Optical pickup apparatus, objective optical element and optical information recording reproducing apparatus

ABSTRACT

An optical pickup apparatus according to the present invention includes: a first light source for emitting a first light flux; a second light source for emitting a second light flux; a third light source for emitting a third light flux; and an objective optical element. The objective optical element has an optical surface including at least two areas provided with optical path difference providing structures. The objective optical element converges the first to third light fluxes each passing through the predetermined areas on the objective optical element onto respective information recording surfaces of the first to third optical disks. One of the optical path difference providing structures includes a first basic structure and a second basic structure which are predetermined optical path difference providing structures and overlap with each other.

This application is based on Japanese Patent Application Nos.2006-060968 filed on Mar. 7, 2006, 2006-070701 filed on Mar. 15, 2006,2006-123471 filed on Apr. 27, 2006, 2006-166148 filed on Jun. 15, 2006,in Japanese Patent Office, the entire content of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical pickup apparatus, objectiveoptical element and optical information recording reproducing apparatusby which information can be recorded and/or reproduced informationcompatibly for different kinds of optical disks.

In recent years, tendency of a shorter wavelength of laser beam as alight source which has been used to record and/or reproduce informationfor optical disks, has become a main stream. For example, a laser lightsource having 400-420 nm wavelength, such as a blue-violet semiconductorlaser; and a blue-SHG laser which converts wavelength of an infraredsemiconductor laser utilizing a second harmonic wave, have been madepractical. Information of 23-27 GB can be recorded on the optical diskhaving a diameter of 12 cm by using these blue-violet optical sourcesand an objective lens having NA (Numerical aperture) which is the sameas a DVD (Digital Versatile Disc). When NA is increased to 0.85,information of 23-25 GB can be recorded onto the optical disk having adiameter of 12 cm. In this specification, the optical disk and anoptical-magnetic disk using a blue-violet laser light source are called“a high density optical disk”.

Hereupon, the high density optical disk using the objective opticalelement, has larger coma caused due to the skew of the optical disk.Therefore some high density optical disks using the objective opticalelement with NA of 0.85, are designed so that the protective layer hasthinner thickness (which is 0.1 mm, while that of DVD is 0.6 mm) thanthat of DVD to reduce the comma due to the skew. On the other hand, itis sometimes considered that a product, such as an optical diskplayer/recorder, which is capable of only recording/reproducinginformation for the above high-density optical disk is worthless. Takingaccount of a fact that, at present, DVDs and CDs (Compact Disc), ontowhich various kinds of information have been recorded, are on themarket, the value of the product as a high-density optical diskplayer/recorder is increased by, for example, enabling to appropriatelyrecord/reproduce information additionally for DVDs and CDs, which a userpossesses. From these backgrounds, the optical pickup apparatusinstalled in the high-density optical disk player/recorder is requiredto be capable of appropriately recording/reproducing information notonly for a high-density optical disk but also a DVD and a CD.

It can be considered, as a method by which the information can beadequately recorded/reproduced while the compatibility is maintainedalso to anyone of the high density optical disk and DVD and further toCD, a method to selectively switch the optical system for the highdensity optical disk and the optical system for DVD and CD correspondingto the recording density of the optical disk to which the information isrecorded/reproduced. However, it requires a plurality of opticalsystems, which is disadvantageous for the size-reduction and whichincreases the cost.

Accordingly, in order to simplify the structure of the optical pickupapparatus and to intend the reduction of cost, it is preferable to makean optical system for the high density optical disk an optical systemfor DVD and CD into a common optical system, and to reduce the number ofoptical parts contributing the optical pickup apparatus as much aspossible, even when the optical pickup apparatus has compatibility.Then, providing the common objective optical element which is arrangedwith facing an optical disk, is most advantageous for the simplificationof the construction or cost reduction of the optical pickup apparatus.Here, in order to obtain the common objective optical element for pluralkinds of optical disks which use different wavelengths forrecording/reproducing information, it is requires that the objectiveoptical system is provided with an optical path difference providingstructure having a wavelength dependency for the spherical aberration,which is formed thereon.

European patent application EP-A 1304689 discloses an objective opticalsystem which has the diffractive structure as an optical path differenceproviding structure and can be commonly used for the high densityoptical disk and the conventional DVD and CD, and also discloses anoptical pickup apparatus in which this objective optical system ismounted.

However, the objective optical element for use in the optical pickupapparatus which compatibly conducts recording and/or reproducinginformation for three different optical disks, which is written in EP-A1304689, has a probability that the light amount used for recordingand/or reproducing information is insufficient, depending on the designspecification of the optical pickup apparatus, or has a problem that theunnecessary light affects a sensor for tracking operation when thetracking operation for CD is conducted, and the accurate trackingoperation is hardly conducted, which is a problem. Particularly, whenusing an infinite optical system for all three different kinds ofoptical disks, in other words, a collimated light flux enters into anobjective optical element, the above problems become remarkable.

SUMMARY OF THE INVENTION

The present invention is provided, with considering the above problems,to achieve at least one of the following objects. The present inventionprovides an optical pickup apparatus, an objective optical element, andan optical information recording reproducing apparatus which canproperly conducts recording and/or reproducing information for threekinds of optical disks having different recording densities such as ahigh density optical disk, DVD and CD, even when using as the objectiveoptical element, and which is capable of simplifying the structure andreducing the cost. In addition, the present invention provides anoptical pickup apparatus, an objective optical element, and an opticalinformation recording reproducing apparatus which can maintain accuracyof the tracking operation for all three different optical disks, evenwhen using an infinite optical system. The present invention provides anoptical pickup apparatus, an objective optical element, and an opticalinformation recording reproducing apparatus which can provide excellenttemperature characteristics and can properly conducts recording and/orreproducing information for the three kinds of disks, even when using aplastic lens for the objective optical element. The present inventionprovides an optical pickup apparatus, an objective optical element, andan optical information recording reproducing apparatus which providesdesired optical characteristics and which can be easily manufacturedwithout using minute structures.

According to various embodiments, the present teachings can provide anoptical pickup apparatus for recording and/or reproducing informationfor an optical disk, an objective optical element for use in the opticalpickup apparatus and an optical information recording reproducingapparatus employing the optical pickup apparatus. The optical pickupapparatus comprises: a first light source for emitting a first lightflux having a first wavelength λ1; a second light source for emitting asecond light flux having a second wavelength λ2 (λ2>λ1); a third lightsource for emitting a third light flux having a third wavelength λ3(λ3>λ2); and an objective optical element. The objective optical elementcan be provided for converging the first light flux onto an informationrecording surface of a first optical disk having a protective substratewith a thickness t1, for converging the second light flux onto aninformation recording surface of a second optical disk having aprotective substrate with a thickness t2 (t1≦t2), and for converging thethird light flux onto an information recording surface of a thirdoptical disk having a protective substrate with a thickness t3 (t2<t3).The optical pickup apparatus can record and/or reproduce information byconverging the first light flux onto the information recording surfaceof the first optical disk, by converging the second light flux onto theinformation recording surface of the second optical disk, and byconverging the third light flux onto the information recording surfaceof the third optical disk. The objective optical element comprises anoptical surface including at least two areas which are a central areaand a peripheral area surrounding the central area. The central areacomprises a first optical path difference providing structure. Theperipheral area comprises a second optical path difference providingstructure. The objective optical element can converge the first lightflux which passes through the central area of the objective opticalelement onto the information recording surface of the first optical diskso that the optical pickup apparatus can record and/or reproduceinformation on the information recording surface of the first opticaldisk. The objective optical element can converge the second light fluxwhich passes through the central area of the objective optical elementonto the information recording surface of the second optical disk sothat the optical pickup apparatus can record and/or reproduceinformation on the information recording surface of the second opticaldisk. The objective optical element can converge the third light fluxwhich passes through the central area of the objective optical elementonto the information recording surface of the third optical disk so thatthe optical pickup apparatus can record and/or reproduce information onthe information recording surface of the third optical disk. Theobjective optical element can converge the first light flux which passesthrough the peripheral area of the objective optical element onto theinformation recording surface of the first optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the first optical disk. The objectiveoptical element can converge the second light flux which passes throughthe peripheral area of the objective optical element onto theinformation recording surface of the second optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the second optical disk. The firstoptical path difference providing structure can comprise a first basicstructure and a second basic structure which overlap with each other.The first basic structure can be an optical path difference providingstructure which emits a second-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the first basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the first basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstbasic structure. The second basic structure can be an optical pathdifference providing structure which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the first light flux passes throughthe second basic structure, which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the second light flux passes throughthe second basic structure, and which emits a ±1^(st) order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the third light flux passesthrough the second basic structure.

The above embodiment allows that one objective optical element toproperly record and/or reproduce information for three different disks(for example, a high density optical disk using a blue-violet laserlight source, DVD, and CD) with employing a simple and low-costconfiguration. Further, the above embodiment allows to provide anoptical pickup apparatus, an objective optical element, and an opticalinformation recording reproducing apparatus which can maintain accuracyof the tracking operation for all three different optical disks, evenwhen using an infinite optical system. The above embodiment furtherallows to provide an optical pickup apparatus, an objective opticalelement, and an optical information recording reproducing apparatuswhich properly record and/or reproduce information for three differentdisks with using a single objective optical element. The aboveembodiment further allow to provide an optical pickup apparatus, anobjective optical element, and an optical information recordingreproducing apparatus which provide excellent temperaturecharacteristics and properly record and/or reproduce information forthree different disks with using a single objective optical element,even when employing a plastic lens.

These and other objects, features and advantages according to thepresent invention will become more apparent upon reading of thefollowing detailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements numbered alike in severalFigures, in which:

FIG. 1 is a diagram showing an example of objective optical element OBJaccording to the present invention viewed from the optical axisdirection;

FIGS. 2( a) to 2(d) are sectional views typically showing severalexamples of the optical path difference providing structure provided inobjective optical element OBJ according to the present invention;

FIG. 3 is a view generally showing the structure of an optical pickupapparatus according to the present invention;

FIG. 4 is a sectional view typically showing an example of objectiveoptical element OBJ according to the present invention;

FIGS. 5( a) to 5(c) are vertical spherical aberration diagrams relatingto BD, DVD, and CD of Example 1 according to the present invention;

FIGS. 6( a) to 6(c) are vertical spherical aberration diagrams relatingto BD, DVD, and CD of Example 2 according to the present invention;

FIGS. 7( a) to 7(c) are vertical spherical aberration diagrams relatingto BD, DVD, and CD of Example 3 according to the present invention;

FIG. 8 is a sectional view typically showing the optical path differenceproviding structure of the objective optical element in Example 4according to the present invention;

FIGS. 9( a) to 9(c) are vertical spherical aberration diagrams relatingto BD, DVD, and CD of Example 4 according to the present invention;

FIG. 10 is a view showing the shape of a spot according to the presentinvention;

FIGS. 11( a) to 11(c) are vertical spherical aberration diagramsrelating to BD, DVD, and CD of Example 5 according to the presentinvention;

FIG. 12 is a view showing the step amount of an example of the opticalpath difference providing structure;

FIG. 13 is a view showing an example of the pitch width;

FIGS. 14( a) to 14(d) are views for illustrating the design method ofthe optical path difference providing structure according to the presentinvention;

FIG. 15 is a perspective view of molded objective optical element OBJ;and

FIG. 16 is a flow chart showing an example of the manufacturing methodof the optical pickup apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An optical pickup apparatus according to the present invention comprisesat least three light sources: a first light source, a second lightsource, and a third light source. The optical pickup apparatus furthercomprises a light converging optical system for converging the firstlight flux on the information recording surface of the first opticaldisk, converging the second light flux on the information recordingsurface of the second optical disk, and converging the third light fluxon the information recording surface of the third optical disk. Theoptical pickup apparatus according to the present invention furthercomprises a light receiving element for receiving each of reflectionlight fluxes from the information recording surface of the first opticaldisk, the second optical disk, or the third optical disk.

The first optical disk comprises a protective substrate with a thicknessof t1 and an information recording surface. The second optical diskcomprises a protective substrate with a thickness of t2 (t1≦t2) and aninformation recording surface. The third optical disk comprises aprotective substrate of a thickness of t3 (t2≦t3) and an informationrecording surface. It is preferable that the first optical disk is ahigh density optical disk, the second optical disk is DVD, and the thirdoptical disk is CD, however, optical disks are not limited to those.Further, in the case where t1<t2, as compared to the case where t1=t2,it is more difficult to record and/or reproduce information for threedifferent optical disks by an objective optical element being a singlelens, with providing excellent tracking characteristics at the time ofrecording and/or reproducing information for the third optical disk.However, an embodiment according to the present invention can conductthat. Hereupon, the first optical disk, the second optical disk or thethird optical disk may also be an optical disk of the plurality oflayers having the plurality of the information recording surfaces.

As an example of the high density optical disk in the presentspecification, there is cited an optical disk (for example, BD: Blue-rayDisc) based on the standard that information is recorded and/orreproduced by an objective optical element with NA 0.85, and that aprotective substrate of the optical disk is about 0.1 mm. Further, as anexample of another high density optical disk, there is cited an opticaldisk (for example, HD DVD: it also called HD) based on the standard thatinformation is recorded and/or reproduced by an objective opticalelement with NA in the range of 0.65 to 0.67 and the protectivesubstrate of the optical disk is about 0.6 mm. Further, the high densityoptical disk includes an optical disk having a protective film (in thepresent specification, the protective substrate includes also theprotective film), having a thickness of about several to several ten nmon the information recording surface, or an optical disk whoseprotective substrate thickness is 0 (zero). The high density opticaldisk further includes a photo-magnetic disk for which the blue-violetsemiconductor laser or blue-violet SHG laser is used as the light sourcefor recording/reproducing information. Further, DVD in the presentspecification represents a generic name of optical disks based on thestandard that information is recorded and/or reproduced by an objectiveoptical element with NA in the range of 0.60 to 0.67 and that theprotective substrate of the optical disc is about 0.6 mm, which belongto DVD group such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R,DVD-RW, DVD+R and DVD+RW. In the present specification, CD represents ageneric name of optical disks based on the standard that information isrecorded and/or reproduced by an objective optical element with NA inthe range of 0.45 to 0.51 and that the protective substrate of theoptical disc is about 1.2 mm, which belong to CD group such as CD-ROM,CD-Audio, CD-Video, CD-R and CD-RW. Among these optical disks, the highdensity optical disk provides the highest recording density. DVD and CDprovide the second highest recording density, the third highestrecording density, respectively.

Thicknesses t1, t2, and t3 of the protective substrates preferablysatisfy the following conditional expressions (6), (7), and (8).However, it is to be understood that various changes and modificationswill be apparent to those skilled in the art.

0.0750 mm≦t1≦0.125 mm or 0.5 mm≦t1≦0.7 mm  (6)

0.5 mm≦t2≦0.7 mm  (7)

1.0 mm≦t3≦1.3 mm  (8)

In the present specification, each of the first light source, the secondlight source, and the third light source is preferably a laser lightsource. A semiconductor laser, and a silicon laser are preferably usedfor the laser light source. The first wavelength λ1 of the first lightflux emitted from the first light source, the second wavelength λ2(λ2>λ1) of the second light flux emitted from the second light source,the third wavelength λ3 (λ3>λ2) of the third light flux emitted from thethird light source, are preferable to satisfy the following conditionalexpressions (9) and (10).

1.5×λ1<λ2<1.7×λ1  (9)

1.9×λ1<λ3<2.1×λ1  (10)

When BD or HD is employed as the first optical disk, the wavelength λ1of the first light source is preferably 350 nm or more, and 440 nm orless. The wavelength λ1 is more preferably 380 nm or more, and 415 nm orless. When DVD is employed as the second optical disk, the secondwavelength λ2 of the second light source is preferably 570 nm or more,and 680 nm or less. The second wavelength λ2 is more preferably 630 nmor more, and 670 nm or less. When CD is employed for the third opticaldisk, the third wavelength λ3 of the third light source is preferably750 nm or more, and 880 nm or less. The third wavelength λ3 is morepreferably 760 nm or more, and 820 nm or less.

Further, at least two light sources of the first light source, thesecond light source, and the third light source may also be unitized.The unitization means fixing and housing, for example, the first lightsource and the second light source into one package. However it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. The unitization in a broad sense includes asituation that two light sources are fixed so that aberration can not becorrected. Further, in addition to the light source, the light receivingelement which will be described later, may also be provided as onepackage.

As the light receiving element, the light detector such as a photo diodeis preferably used. The light reflected on the information recordingsurface of the optical disk enters into the light receiving element, andsignal outputted from the light receiving element is used for obtainingthe read signal of the information recorded in each optical disk.Further, the light amount of the spot on the light receiving elementcaused with the change in the spot shape and the change in the spotposition, to conduct the focus detection and the tracking detectionfocus detection. The objective optical element is moved based on thesedetections for focusing and tracking of the objective optical element.The light receiving element may be composed of a plurality of lightdetectors. The light receiving element may also have a main lightdetector and secondary light detector. For example, the light receivingelement is provided with a main light detector which receives the mainlight used for recording and/or reproducing information, and twosecondary light detectors positioned on both sides of the main lightdetector, so as to receive secondary light for tracking adjustment bythe two secondary light detectors. Further, the light receiving elementmay also comprise a plurality of light receiving elements correspondingto each light source.

The light converging optical system comprises the objective opticalelement. The light converging optical system may comprise only anobjective optical element, however, the light converging optical systemmay also have a coupling lens such as a collimator lens other than theobjective optical element. The coupling lens is arranged between theobjective optical element and the light source and means a single lensor a lens group which changes divergent angle of a light flux. Thecollimator lens is a lens to collimate the light flux entered into thecollimator lens. Further, the light converging optical system may alsocomprise an optical element such as the diffractive optical elementwhich divides the light flux emitted from the light source into a mainlight flux used for recording reproducing information and two secondarylight fluxes used for the tracking operation. In the presentspecification, the objective optical element means an optical systemwhich is arranged to face the optical disk in the optical pickupapparatus, which has the function which converges the light flux emittedfrom the light source onto an information recording surface of theoptical disk. Preferably, the objective optical element is an opticalsystem which is arranged to face the optical disk in the optical pickupapparatus, and which has the function which converges the light fluxemitted from the light source on the information recording surface ofthe optical disk, and further which is movable as one body in thedirection of at least the optical axis by an actuator. The objectiveoptical element may be formed of a plurality of lenses and/or opticalelements. Alternatively, the objective optical element may be a singlelens. Preferably, the objective lens is formed of a single lens. Theobjective optical element may also be a glass lens, a plastic lens or ahybrid lens in which an optical path difference providing structure isformed on the glass lens by using thermosetting resin. When theobjective optical element has a plurality of lenses, a combination of aglass lens and a plastic lens can be used for the objective opticalelement. When the objective optical element has a plurality of lensesand/or optical elements, a combination of: an optical element in flatplate shape having an optical path difference providing structure; andan aspheric surface lens which may not have a optical path differenceproviding structure. The objective optical element comprises arefractive surface which is an aspheric surface. Further, the objectiveoptical element preferably can have a base surface where the opticalpath difference providing structure is provided, which is an asphericsurface.

Further, when the objective optical element is a glass lens, a glassmaterial used for the glass lens preferably has a glass transition pointTg of 400° C. or less. By using the glass material whose glasstransition point Tg is 400° C. or less, the material can be molded at acomparatively low temperature. Therefore, the life of the metallic moldcan be prolonged. As an example of the glass material whose glasstransition point Tg is low, there are K-PG325 and K-PG375 (both aretrade names) made by SUMITA Optical glass, Inc.

Hereupon, a glass lens has generally larger specific gravity than aresin lens. Therefore, the objective optical element made of a glasslens has larger weight and apply a larger burden to the actuator whichdrives the objective optical element. Therefore, when a glass lens isemployed for the objective optical lens, a glass material having smallspecific gravity is preferably used for the objective optical element.Specifically, the specific gravity is preferably 3.0 or less, and ismore preferably 2.8 or less.

Further, when a plastic lens is employed for the objective opticalelement, it is preferable that the resin material of cyclic olefins isused for the objective optical element. In the cyclic olefins, there ismore preferably used the resin material having: refractive index at thetemperature 25° C. for wavelength 405 nm, which is within the range of1.54 to 1.60; and ratio of refractive index change dN/dT (° C.⁻¹) withthe temperature change within the temperature range of −5° C. to 70° C.for the wavelength 405 nm, which is within the range of −20×10⁻⁵ to−5×10⁻⁵ (more preferably, −10×10⁻⁵ to −8×10⁻⁵). Further, when a plasticlens is employed for the objective optical element, it is preferablethat a plastic lens is also employed for the coupling lens.

Alternatively, as the resin material appropriate to the objectiveoptical element of the present invention, there is “athermal resin” alsoother than the cyclic olefins. “Athermal resin” is a resin material inwhich microparticles each having a diameter of 30 nm or less aredispersed into a resin which is a base material. The material of themicroparticles has ratio of change in the refractive index with thetemperature change, and the ratio of change in the refractive index ofthe microparticles has the opposite sign to that of the material of theresin which is the base material. Generally, when microparticles aremixed in the transparent resin material, light is scattered and thetransmission factor is lowered. So, it is difficult to use as theoptical material. However, it becomes clear that the microparticleswhose size is smaller than the wavelength of the transmitting light fluxprevent the scattering effectively.

Hereupon, the refractive index of the resin material is lowered when thetemperature rises, while the refractive index of the inorganicmicroparticles is increased when the temperature rises. Accordingly, itis also well known to prevent the refractive index from changing bycombining above nature of the microparticles and the base material so asto cancel them out each other. When the objective optical elementaccording to the present invention employs the material such that theinorganic particles whose size is 30 nanometer or less, which ispreferably 20 nanometer or less, more preferably 10-15 nanometer, aredispersed in the resin as base material, there can be provided theobjective optical unit having no or very low temperature dependency ofthe refractive index.

For example, microparticles of niobium oxide (Nb₂O₅) are dispersed inacryl resin. The volume ratio of the resin material that represents thebasic material is about 80% and that of niobium oxide is about 20%, andthese are mixed uniformly. Though microparticles have a problem thatthey tend to condense, the necessary state of dispersion can be kept bya technology to disperse particles by giving electric charges to thesurface of each particle.

It is preferable that microparticles are mixed and dispersed into theresin as a base material during injection molding of optical elements bythe in-line manner. In other words, it is preferable that, after themicroparticles are mixed and dispersed in to the base material, themixture is neither cooled nor solidified until the mixture is moldedinto an objective optical unit.

Incidentally, in order to control the ratio of change in the refractiveindex with the temperature, the volume ratio of microparticles to thebase material may increase or decrease, and microparticles in which aplural kinds of nanometer-sized microparticles are blended may also bedispersed into the base material.

Though the volume ratio of the microparticles and the base material ismade to be 80:20, namely to be 4:1, in the example stated above, it ispossible to adjust properly within a range from 90:10 (9:1) to 60:40(3:2). It is preferable that a volume of the microparticles is providedto be exceed the ratio of 9:1, because the temperature-affected changeis effectively reduced. While, it is also preferable that a volume ofthe microparticles is provided to be less than the ratio of 3:2, becausemoldability of the athermal resin becomes easy.

It is preferable that the microparticles are inorganic substances, andmore preferable that the microparticles are oxides. Further, it ispreferable that the state of oxidation is saturated, and the oxides arenot oxidized any more.

It is preferable that the microparticles are inorganic substancesbecause reaction between the inorganic substances and resin as a basematerial representing high molecular organic compound is restrained tobe low, and deterioration caused by actual use such as irradiation oflaser beam can be prevented because the microparticles are oxides. Inparticular, under the severe conditions such as high temperature andirradiation of a laser beam, oxidation of resin tends to be accelerated.However, aforesaid microparticles of inorganic oxide are prevented fromdeterioration caused by oxidation.

Further, it is naturally possible to add antioxidants in resin materialin order to prevent the resin from oxidation caused by other factors.

Materials described in JP-A 2004-144951, JP-A 2004-144953, JP-A2004-144954 are suitable for a preferable material to be base material.

Inorganic microparticles to be dispersed in thermoplastic resin are notlimited in particular, and suitable microparticles can be arbitrarilyselected from inorganic microparticles which reduce the ratio(hereinafter, |dn/dT|) of change in refractive index with thetemperature. To be concrete, oxide microparticles, metal saltmicroparticles and semiconductor microparticles are preferably used, andit is preferable to use by selecting properly those in which absorption,light emission and fluorescence are not generated in the wavelengthrange employed for an optical element, from the aforesaidmicroparticles.

The following metal oxide is used for oxide microparticles used in thestructure according to the present invention: a metal oxide constructedby one or more kinds of metal selected by a group including Li, Na, Mg,Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr, Y, Nb, Zr,Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ta, Hf, W, Ir, Tl, Pb, Bi and rareearth metal. More specifically, for example, oxide such as siliconoxide, titanium oxide, zinc oxide, aluminum oxide, zirconium oxide,hafnium oxide, niobium oxide, tantalum oxide, magnesium oxide, calciumoxide, strontium oxide, barium oxide, indium oxide, tin oxide, leadoxide; complex oxide compounds these oxides such as lithium niobate,potassium niobate and lithium tantalate, the aluminum magnesium oxide(MgAl₂O₄) are cited. Furthermore, rare earth oxides are used for theoxide microparticles in the structure according to the presentinvention. More specifically, for example, scandium oxide, yttriumoxide, lanthanum trioxide, cerium oxide, praseodymium oxide, neodymiumoxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide,dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbiumoxide, lutetium oxide are cited. As metal salt microparticles, thecarbonate, phosphate, sulfate, etc. are cited. More specifically, forexample, calcium carbonate, aluminum phosphate are cited.

Moreover, semiconductor microparticles in the structure according to thepresent invention mean the microparticles constructed by asemiconducting crystal. The semiconducting crystal composition examplesinclude simple substances of the 14th group elements in the periodictable such as carbon, silica, germanium and tin; simple substances ofthe 15th group elements in the periodic table such as phosphor (blackphosphor); simple substances of the 16th group elements in the periodictable such as selenium and tellurium; compounds comprising a pluralnumber of the 14th group elements in the periodic table such as siliconcarbide (SiC); compounds of an element of the 14th group in the periodictable and an element of the 16th group in the periodic table such as tinoxide (IV) (SnO₂), tin sulfide (II, IV) (Sn(II)Sn(IV)S₃), tin sulfide(IV) (SnS₂), tin sulfide (II) (SnS), tin selenide (II) (SnSe), tintelluride (II) (SnTe), lead sulfide (II) (PbS), lead selenide (II)(PbSe) and lead telluride (II) (PbTe); compounds of an element of the13th group in the periodic table and an element of the 15th group in theperiodic table (or III-V group compound semiconductors) such as boronnitride (BN), boron phosphide (BP), boron arsenide (BAs), aluminumnitride (AlN), aluminum phosphide (AlP), aluminum arsenide (AlAs),aluminu antimonide (AlSb), gallium nitride (GaN), gallium phosphide(GaP), gallium arsenide (GaAs), gallium antimonide (GaSb), indiumnitride (InN), indium phophide (InP), indium arsenide (InAs) and indiumantimonide (InSb); compounds of an element of the 13th group in theperiodic table and an element of the 16th group in the periodic tablesuch as aluminum sulfide (Al₂S₃), aluminum selenide (Al₂Se₃), galliumsulfide (Ga₂S₃), gallium selenide (Ga₂Se₃), gallium telluride (Ga₂Te₃),indium oxide (In₂O₃), indium sulfide (In₂S₃), indium selenide (In₂Se₃)and indium telluride (In₂Te₃); compounds of an element of the 13th groupin the periodic table and an element ot the 16th group in the periodictable such as thallium chloride (I) (TlCl), thallium bromide (I) (TlBr),thallium iodide (I) (TlI); compounds of an element of the 12th group inthe periodic table and an element of the 16th group in the periodictable (or II-VI group compound semiconductors) such as zinc oxide (ZnO),zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmiumoxide (CdO), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmiumtelluride (CdTe), mercury sulfide (HgS), mercury selenide (HgSe) andmercury telluride (HgTe); compounds of an element of the 15th group inthe periodic table and an element of the 16th group in the periodictable such as arsenic sulfide (III) (As₂S₃), arsenic selenide (III)(As₂Se₃), arsenic telluride (III) (As₂Te₃), antimony sulfide (III)(Sb₂S₃), antimony selenide (III) (Sb₂Se₃), antimony telluride (III)(Sb₂Te₃), bismuth sulfide (III) (Bi₂S₃), bismuth selenide (III) (Bi₂Se₃)and bismuth telluride (III) (Bi₂Te₃); compounds of an element of the11th group in the periodic table and an element of the 16th group in theperiodic table such as copper oxide (I) (Cu₂O) and copper selenide (I)(Cu₂Se); compounds of an element of the 11th group in the periodic tableand an element of the 17th group in the periodic table such as copperchloride (I) (CuCl), copper bromide (I) (CuBr), copper iodide (I) (CuI),silver chloride (AgCl) and silver bromide (AgBr); compounds of anelement of the 10th group in the periodic table and an element of the16th group in the periodic table such as nickel oxide (II) (NiO);compounds of an element of the 9th group in the periodic table and anelement of the 16th group in the periodic table such as cobalt oxide(II) (CoO) and cobalt sulfide (II) (CoS); compounds of an element of the8th group in the periodic table and an element of the 16th group in theperiodic table such as triiron tetraoxide (Fe₃O₄) and iron sulfide (II)(FeS); compounds of an element of the 7th group in the periodic tableand an element of the 16th group in the periodic table such as manganeseoxide (II) (MnO); compounds of an element of the 6th group in theperiodic table and an element of the 16th group in the periodic tablesuch as molybdenum sulfide (IV) (MoS₂) and tungsten oxide (IV) (WO₂);compounds of an element of the 5th group in the periodic table and anelement of the 16th group in the periodic table such as vanadium oxide(II) (VO), vanadium oxide (IV) (VO₂) and tantalum oxide (V) (Ta₂O₅);compounds of an element of the 4th group in the periodic table and anelement of the 16th group in the periodic table such as titanium oxide(such as TiO₂, Ti₂O₅, Ti₂O₃ and Ti₅O₉); compounds of an element of the2th group in the periodic table and an element of the 16th group in theperiodic table such as magnesium sulfide (MgS) and magnesium selenide(MgSe); chalcogen spinels such as cadmium oxide (II) chromium (III)(CdCr₂O₄), cadmium selenide (II) chromium (III) (CdCr₂Se₄), coppersulfide (II) chromium (III) (CuCr₂S₄) and mercury selenide (II) chromium(III) (HgCr₂Se₄); and barium titanate (BaTiO₃). Further, semiconductorclusters structures of which are established such as BN₇₅(BF₂)₁₅F₁₅,described in Adv. Mater., vol. 4, p. 494 (1991) by G. Schmid, et al.;and Cu₁₄₆Se₇₃(triethylphosphine)₂₂ described in Angew. Chem. Int. Ed.Engl., vol. 29, p. 1452 (1990) by D. Fenske are also listed as examples.

In general, dn/dT of thermoplastic resin has a negative value, namely, arefractive index becomes smaller as the temperature rises. Therefore, itis preferable to disperse microparticles having large dn/dT, in order tomake |dn/dT| of thermoplastic resin composition to be efficiently small.It is preferable that the absolute value of dn/dT of the microparticlesis smaller than that of the thermoplastic resin used as a base materialwhen using microparticles having dn/dT with same sign to the sign ofdn/dT of the thermoplastic resin. Furthermore, microparticles havingpositive dn/dT, which is microparticles having different sign of dn/dTfrom that of the thermoplastic resin which is a base material, arepreferably used. By dispersing these kinds of microparticles into thethermoplastic resin, |dn/dT| of thermoplastic resin composition caneffectively become small with less amount of the microparticles. It ispossible to properly select dn/dT of microparticles to be dispersedcorresponding to a value of dn/dT of thermoplastic resin to become abase material. However, it is preferable that dn/dT of microparticles isgreater than −20×10⁻⁶ and it is more preferable that dn/dT ofmicroparticles is greater than −10×10⁻⁶ when microparticles aredispersed into a thermoplastic resin which is preferably employed to ageneral optical element. As microparticles having large dn/dT, galliumnitride, zinc sulfate, zinc oxide, lithium niobate and lithiumtantalite, for example, are preferably used.

On the other hand, when dispersing microparticles in thermoplasticresin, it is preferable that a difference of refractive index betweenthe thermoplastic resin to become a base material and the microparticlesis small. Scattering is hardly caused when light is transmitted, if adifference of refractive index between the thermoplastic resin and themicroparticles to be dispersed is small. In case of dispersingmicroparticles in the thermoplastic resin, microparticles in larger sizeeasily cause scattering when light flux transmits the material. However,in a material in which a difference of refractive index between thethermoplastic resin and the microparticles to be dispersed is small, anoccurrence of light scattering becomes low even when relativelylarge-sized microparticles are used. A difference of refractive indexbetween the thermoplastic resin and the microparticles to be dispersedis preferably within the range of 0-0.3, and more preferably within therange of 0-0.15.

Refractive indexes of thermoplastic resins preferably used as opticalmaterials are in the range about 1.4-1.6 in many cases. As materials tobe dispersed in these thermoplastic resins, silica (silicon oxide),calcium carbonate, aluminum phosphate, aluminum oxide, magnesium oxide,and aluminum•magnesium oxides, for example, are preferably used.

Further, dn/dT of thermoplastic resin composition can be made smalleffectively, by dispersing microparticles whose refractive index isrelatively low. As a reason why |dn/dT| of thermoplastic resincomposition including dispersed microparticles with low refractive indexbecomes small, it is considered that temperature changes of the volumefraction of inorganic microparticles in the resin composition may workto make the |dn/dT| of the resin composition to become smaller when therefractive index of the microparticles is lower, although the detailsare not clarified. As microparticles having a relatively low refractiveindex, silica (silicon oxide), calcium carbonate and aluminum phosphate,for example, are preferably used.

It is difficult to simultaneously achieve all of improving an effect oflowering dn/dT of the thermoplastic resin composition, improving oflight transmittance and a desired refractive index. Therefore,microparticles to be dispersed in the thermoplastic resin can beselected properly by considering a magnitude of dn/dT of a microparticleitself, a difference of dn/dT between microparticles and thethermoplastic resin to become a base material, and the refractive indexof the microparticles, depending on the characteristics which arerequired for the thermoplastic resin composition. Further, it ispreferable, for maintaining light transmittance, to properly selectmicroparticles which hardly cause light scattering with considering itsaffinity with the thermoplastic resin to become a base material, inother words, characteristics of the microparticles in dispersion for thethermoplastic resin.

For example, when using cyclic olefin polymer preferably employed for anoptical element as a base material, silica is preferably used asmicroparticles which make |dn/dT| small while keeping lighttransmittance.

For the microparticles mentioned above, it is possible to use either onetype of inorganic microparticles or plural types of inorganicmicroparticles in combination. By using plural types of microparticleseach having a different characteristic, the required characteristics canfurther be improved efficiently.

Inorganic microparticles relating to the present invention preferablyhas an average particle size being 1 nm or larger and being 30 nm orsmaller and more preferably has an average particle size being 1 nm ormore and being 10 nm or less. When the average particle size is lessthan 1 nm, dispersion of the inorganic microparticles is difficult,resulting in a fear that the required efficiency may not be obtained,therefore, it is preferable that the average particle size is 1 nm ormore. When the average particle size exceeds 30 nm, thermoplasticmaterial composition obtained becomes muddy and transparency is lowered,resulting in a fear that the light transmittance may become less than70%, therefore, it is preferable that the average particle size is 30 nmor less. The average particle size mentioned here means volume averagevalue of a diameter (particle size in conversion to sphere) inconversion from each particle into a sphere having the same volume asthat of the particle.

Further, a form of an inorganic microparticle is not limited inparticular, but a spherical microparticle is used preferably. To beconcrete, a range of 0.5-1.0 for the ratio of the minimum size of theparticle (minimum value of the distance between opposing two tangentseach touching the outer circumference of the microparticle)/the maximumsize (maximum value of the distance between opposing two tangents eachtouching the outer circumference of the microparticle) is preferable,and a range of 0.7-1.0 is more preferable.

A distribution of particle sizes is not limited in particular, but arelatively narrow distribution is used suitably, rather than a broaddistribution, for making the invention to exhibit its effectefficiently.

The objective optical element will be described below. At least oneoptical surface of the objective optical element comprises a centralarea and a peripheral area around the central area. More preferably, atleast one optical surface of the objective optical element furtherincludes a most peripheral area around the peripheral area. By providingthe most peripheral area, it allows to more appropriately record and/orreproduce information for the optical disk using the high NA. Thecentral area preferably is an area having the optical axis of theobjective optical element, however, it may also be the area notincluding the optical axis. It is preferable that the central area,peripheral area, and most peripheral area are provided on the sameoptical surface. As shown in FIG. 1, it is preferable that the centralarea CN, peripheral area MD, most peripheral area OT are provided on thesame optical surface concentrically around the optical axis. Further,the first optical path difference providing structure is provided in thecentral area of the objective optical element. The second optical pathdifference providing structure is provided in the peripheral area. Whenthe most peripheral area is provided, the most peripheral area may be arefractive surface, or the third optical path difference providingstructure may be provided in the most peripheral area. It is preferablethat each of the central area, peripheral area, most peripheral areaadjoins to the neighboring area, however, there may be slight gapsbetween adjoining areas.

The area where the first optical path difference providing structure isprovided is preferably 70% or more of the area of the central area onthe objective optical element. It is more preferably 90% or more of thearea of the central area. The first optical path difference providingstructure is furthermore preferably provided on the entire surface ofthe central area. The area where the second optical path differenceproviding structure is provided is preferably 70% or more of theperipheral area on the objective optical element. It is more preferably90% or more of the area of the peripheral area. The second optical pathdifference providing structure is further more preferably provided onthe entire surface of the peripheral area. The area where the thirdoptical path difference providing structure is provided, is 70% or moreof the area of the most peripheral area on the objective opticalelement. It is more preferably 90% or more of the area of the mostperipheral area. The third optical path difference providing structureis more preferably provided on the entire surface of the most peripheralarea.

Hereupon, the optical path difference providing structure used in thepresent specification, is the general name of the structure by which anoptical path difference is provided to an incident light flux. Theoptical path difference providing structure also includes the phasedifference providing structure by which the phase difference isprovided. Further, the phase difference providing structure includes adiffractive structure. The optical path difference providing structurehas a step, preferably, has a plurality of steps. This step provides anoptical path difference and/or phase difference to an incident lightflux. The optical path difference added by the optical path differenceproviding structure may also be an integer times of the wavelength ofthe incident light flux, or may also be non-integer times of thewavelength of the incident light flux. The step may also be arrangedwith periodic interval in the direction perpendicular to the opticalaxis, or may also be arranged with non-periodic interval in thedirection perpendicular to the optical axis.

It is preferable that the optical path difference providing structurehas a plurality of ring-shaped zones arranged concentrically around theoptical axis. Further, the optical path difference providing structurecan have various sectional shapes (cross sectional shapes in the planeincluding the optical axis). One of the most common optical pathdifference providing structure provides the sectional shape includingthe optical axis, which is in the serrated shape, as shows in FIG. 2(a). Even when the cross sectional shape of the optical path differenceproviding structure arranged on the flat plane looks a stepped shape,the same optical path difference providing structure arranged on anaspheric surface can be considered as the serrated shape shown in FIG.2( a). Accordingly, in the present specification, it is defined that thesectional shape in the serrated shape includes the sectional shape inthe stepped shape. Further, when the serrated shaped optical pathdifference providing structures whose step extend to the differentdirections, overlap with each other, there is provided the binarystructured optical path difference providing structure as shown in FIG.2( b). Each of the first optical path difference providing structure andthe second optical path difference providing structure of the presentspecification, may have a sectional shape which is formed by overlappingdifferent optical path difference providing structures having serratedshape, or may have a sectional shape which is formed by furtheroverlapping a serrated shaped optical path difference providingstructure with a binary structured optical path difference providingstructure formed by overlapping serrated optical path differenceproviding structures. For example, FIG. 2( c) shows the structure inwhich the serrated shaped optical path difference providing structureand the binary structured optical path difference providing structureoverlap with each other, and FIG. 2( d) shows the structure in which thefine serrated shaped optical path difference providing structure and therough serrated optical path difference providing structure overlap witheach other.

Further, the first optical path difference providing structure providedin the central area of the objective optical element and the secondoptical path difference providing structure provided in the peripheralarea of the objective optical element may be provided on the differentoptical surface of the objective optical element. However, it ispreferable that the first and second optical path difference providingstructures are provided on the same optical surface. By providing themon the same optical surface, it reduces the decentration error at thetime of the manufacture, which is preferable. Further, it is preferablethat the first optical path difference providing structure and thesecond optical path difference providing structure are provided on thesurface on the light source side of the objective optical element,rather than the surface on the optical disk side of the objectiveoptical element.

The objective optical element converges the first light flux, the secondlight flux, and the third light flux each passing through the centralarea of the objective lens, where the first optical path differenceproviding structure is provided, so as to form a light converged spotfor each light flux. Preferably, the objective optical element convergesthe first light flux passing through the central area of the objectiveoptical element where the first optical path difference providingstructure is provided onto the information recording surface of he firstoptical disk, so that information can be recorded and/or reproduced forthe first optical disk. Preferably, the objective optical elementconverges the second light flux passing the central area of theobjective optical element where the first optical path differenceproviding structure is provided onto the information recording surfaceof the second optical disk, so that information can be recorded and/orreproduced for the second optical disk. Preferably, the objectiveoptical element converges the third light flux passing the central areaof the objective optical element where the first optical path differenceproviding structure is provided onto the information recording surfaceof the third optical disk, so that information can be recorded and/orreproduced for the third optical disk. Further, when the thickness t1 ofthe protective substrate of the first optical disk and the thickness t2of the protective substrate of the second optical disk are different,the first optical path difference providing structure preferablycorrects the spherical aberration caused due to the difference betweenthe thickness t1 of the protective substrate of the first optical diskand the thickness t2 of the protective substrate of the second opticaldisk and/or the spherical aberration caused due to the difference of thewavelength of the first light flux and the second light flux, for thefirst light flux and the second light flux passing through the firstoptical path difference providing structure. Furthermore, the firstoptical path difference providing structure preferably corrects thespherical aberration caused due to the difference between the thicknesst1 of the protective substrate of the first optical disk and thethickness t3 of the protective substrate of the third optical diskand/or the spherical aberration caused due to the difference of thewavelength of the first light flux and the third light flux, for thefirst light flux and the third light flux passing through the firstoptical path difference providing structure.

Further, the third light flux passing through the first optical pathdifference providing structure of the objective optical element, forms afirst best focus in which the third light flux forms a spot having thesmallest diameter, and a second best focus, in which the third lightflux forms a spot having the second smallest diameter. Hereupon, thebest focus used herein means a position where the beam waist isminimized within the predetermined defocus range. That is, the case thatthe third light flux forms the first best focus and the second bestfocus means that the beam waist in the third light flux is minimizedwithin the predetermined defocus range on at least two positions.Hereupon, the first best focus is preferably formed by a diffractedlight flux with a maximum light amount in the third light flux passingthrough the first optical path difference providing structure. Thesecond best focus is preferably formed by a diffracted light flux with asecond maximum light amount in the third light flux passing through thefirst optical path difference providing structure. When the differencebetween the diffraction efficiency of the diffracted light flux whichforms the first best focus and the diffraction efficiency of thediffracted light flux which forms the second best focus, is 20% or less,the effect of the present invention becomes more conspicuous.

Hereupon, it is preferable that the spot formed by the third light fluxin the first best focus is used for recording and/or reproducinginformation for the third optical disk, and that the spot formed by thethird light flux in the second best focus is not used for recordingand/or reproducing information for the third optical disk. However, itdoes not denies an embodiment that the spot formed by the third lightflux in the first best focus is not used for recording and/orreproducing information for the third optical disk, and that the spotformed by the third light flux in the second best focus is used forrecording and/or reproducing information for the third optical disk.Hereupon, when the first optical path difference providing structure isprovided on the surface on the light source side of the objectiveoptical element, the second best focus is preferably nearer to theobjective optical element, than the first best focus.

Further, the first best focus and the second best focus satisfy thefollowing expression (1).

0.05≦L/f≦0.35  (1)

Where, f (mm) indicates the focal length of the objective opticalelement for the third light flux which passes through the first opticalpath difference providing structure and forms the first best focus, andL (mm) indicates the distance between the first best focus and thesecond best focus.

The first best focus and the second best focus more preferably satisfythe following expression (1′).

0.10≦L/f≦0.25  (1′)

The first best focus and the second best focus further more preferablysatisfy the following expression (1″).

0.11≦L/f≦0.24  (1″)

Further, it is preferable that L is 0.18 mm or more, and is 0.63 mm orless. Furthermore, it is preferable that f is 1.8 mm or more, and is 3.0mm or less.

In case that the above expression (1) is satisfied, it allows to preventthe unused light which is a light not used for recording and/orreproducing information for the third optical disk, from affecting thelight receiving element for the tracking operation, and enables tomaintain the excellent tracking characteristics when information isrecorded and/or reproduced for the third optical disk.

Further, the objective optical element converges the first light fluxand the second light flux each passing through the peripheral area ofthe objective optical element, where the second optical path differenceproviding structure is provided, so as to form a light converged spotfor each light flux. Preferably, the objective optical element convergesthe first light flux passing through the peripheral area of theobjective optical element where the second optical path differenceproviding structure is provided onto the information recording surfaceof the first optical disk so that information can be recorded and/orreproduced for the first optical disk. Preferably, the objective opticalelement converges the second light flux passing through the peripheralarea of the objective optical element where the second optical pathdifference providing structure is provided, onto the informationrecording surface of the second optical disk so that information can berecorded and/or reproduced for the second optical disk. Further, whenthe thickness t1 of the protective substrate of the first optical diskand the thickness t2 of the protective substrate of the second opticaldisk are different, the second optical path difference providingstructure preferably corrects the spherical aberration caused due to thedifference between the thickness t1 of the protective substrate of thefirst optical disk and the thickness t2 of the protective substrate ofthe second optical disk, and/or the spherical aberration caused due tothe difference of the wavelength of the first light flux and the secondlight flux, for the first flux and the second light flux passing throughthe second optical path difference providing structure.

Further, there is provided one preferable embodiment that the thirdlight flux passing through the peripheral area is not used for recordingand/or reproducing information for the third optical disk. In theembodiment, it is preferable that the third light flux passing throughthe peripheral area does not contribute to forming the light convergedspot on the information recording surface of the third optical disk. Inother words, it is preferable that the third light flux passing throughthe peripheral area comprising the second optical path differenceproviding structure of the objective optical element, forms flare on theinformation recording surface of the third optical disk. FIG. 10 showsthe spot formed by the third light flux passing through the objectiveoptical element on the information recording surface of the thirdoptical disk. The spot shown in FIG. 10 comprises, in order from theoptical axis side (or the central part of the spot) to the outside ofthe spot, the central spot portion SCN whose light density is high, theintermediate spot portion SMD whose light density is lower than thecentral spot portion, and the peripheral spot portion SOT whose lightdensity is higher than the intermediate spot portion and lower than thecentral spot portion. The central spot portion is used for recordingand/or reproducing information for an optical disk, and the intermediatespot portion and the peripheral spot portion are not used for recordingand/or reproducing information for a optical disk. In the abovedescription, this peripheral spot portion can be called flare. That is,the third light flux passing through the second optical path differenceproviding structure provided in the peripheral area of the objectiveoptical element, forms the peripheral spot portion on the informationrecording surface of the third optical disk. Hereupon, it is preferablethat the light converging spot or the spot of the third light flux inthe above is a spot in the first best focus. Further, it is preferablethat the second light flux passing through the objective opticalelement, forms a spot on the information recording surface of the secondoptical disk, which comprises a central spot portion, an intermediatespot portion, and a peripheral spot portion.

Further, it is preferable that the second optical path differenceproviding structure corrects the spherochromatism (chromatic sphericalaberration) caused due to the slightly fluctuating wavelength of thefirst light source or the second light source. The slight fluctuation ofthe wavelength means the fluctuation within ±10 nm. For example, whenthe first light flux changes by ±5 nm from the wavelength λ1, it ispreferable that the second optical path difference providing structurecorrects the fluctuation of the spherical aberration of the first lightflux passing through the peripheral area and the amount of thefluctuation of the spherical aberration on the information recordingsurface of the first optical disk is 0.010 λ1 rms or more, and is 0.095λ1 rms or less. Further, when the second light flux changes by ±5 nmfrom the wavelength λ2, it is preferable that the second optical pathdifference providing structure corrects the fluctuation of the sphericalaberration of the second light flux passing through the peripheral areaand the amount of the fluctuation of the spherical aberration on theinformation recording surface of the second optical disk is 0.002 λ2 rmsor more, and is 0.03 λ2 rms or less. Therefore, the aberration due tothe fluctuation of the wavelength by the manufacturing error of thewavelength of the laser diode which is a light source, or the individualdifference of the laser diode can be corrected.

When the objective optical element comprises the most peripheral area,the objective optical element converges the first light flux passingthrough the most peripheral area of the objective optical element, ontothe information recording surface of the first optical disk so thatinformation can be recorded and/or reproduced for the first opticaldisk. Further, the spherical aberration in the first light flux passingthrough the most peripheral area, is preferably corrected when recordingand/or reproducing information for the first optical disk is conducted.

Further, there is provided a preferable embodiment that the second lightflux passing through the most peripheral area is not used for recordingand/or reproducing information for the second optical disk, and that thethird light flux passing through the most peripheral area is not usedfor recording and/or reproducing information for the third optical disk.In the embodiment, it is preferable that the second light flux and thethird light flux each passing through the most peripheral area do notcontribute to forming the light converged spots on respectiveinformation recording surface of the second optical disk and the thirdoptical disk. That is, when the objective optical element comprises themost peripheral area, the third light flux passing through the mostperipheral area of the objective optical element preferably forms flareon the information recording surface of the third optical disk. In otherwords, the third light flux passing through the most peripheral area ofthe objective optical element, preferably forms the peripheral spotportion on the information recording surface of the third optical disk.Further, when the objective optical element comprises the mostperipheral area, the second light flux passing the most peripheral areaof the objective optical element preferably forms flare on theinformation recording surface of the second optical disk. In otherwords, the second light flux which has passed through the mostperipheral area on the objective optical element, preferably forms theperipheral spot portion on the information recording surface of thesecond optical disk.

When the most peripheral area comprises the third optical pathdifference providing structure, the third optical path differenceproviding structure may correct the sperochromatism (chromatic sphericalaberration) caused due to the slightly fluctuated wavelength of thefirst light source. The slight fluctuation of the wavelength means afluctuation within ±10 nm. For example, when the first light fluxchanges by ±5 nm from the wavelength λ1, it is preferable that the thirdoptical path difference providing structure corrects the fluctuation ofthe spherical aberration of the first light flux passing through theperipheral area and the amount of the fluctuation of the sphericalaberration on the information recording surface of the first opticaldisk is 0.010 λ1 rms or more, and is 0.095 λ1 rms or less.

Hereupon, the first optical path difference providing structure may bethe structure in which the serrated shaped diffractive structure and thebinary structure overlap with each other. Further, the second opticalpath difference providing structure may be the structure in which theserrated shaped diffractive structure and the rougher (its pitch islarger) serrated shaped diffractive structure overlap with each other.When the first optical path difference providing structure is theoverlapped structure, the serrated shaped diffractive structure mayprovide optical path difference corresponding to the even times of thefirst wavelength λ1 of the first light flux, so as not to provide achange in a phase of the wavefront of the first light flux. When thesecond optical path difference providing structure is the overlappedstructure, the finer serrated shaped diffractive structure (with smallerpitch) may provide optical path difference corresponding to the eventimes of the first wavelength λ1 of the first light flux, so as not toprovide a change in a phase of the wavefront of the first light flux.Further, when the third wavelength λ3 of the third light flux is thewavelength which is about even times of the first wavelength of thefirst light flux, the integer times optical path difference is providedwith the third light flux and it does not cause phase change in thewavefront of the third light flux. This structure provides an advantagethat the above diffractive structure does not affect converging lightspot of the first light flux and the third light flux. Hereupon, a valuecorresponding to the even times indicates a value which is (2n −0.1)×λ1or more, and is (2n+0.1)×λ1 or less, where n is a natural number.

The first optical path difference providing structure may be thestructure in which at least the first basic structure and the secondbasic structure overlap with each other.

The first basic structure is the optical path difference providingstructure which emits a second-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the first basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the first basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstbasic structure. The first basic structure is preferably the opticalpath difference providing structure which emits the first light flux andthe third light flux each of which passes through the first basicstructure with their wavefront substantially being even, and which emitsthe second light flux which passes through the first basic structurewith the wavefront substantially being uneven. Further the first basicstructure is preferably the optical path difference providing structurewhich makes a diffraction angle of the second light flux passing throughthe first basic structure different from the diffraction angles of thefirst light flux and the third light flux each passing through the firstbasic structure. The step amount in the direction of the optical axis ofthe step in the first basic structure is preferably the step amountproviding optical path difference which is about two times of the firstwavelength for the first light flux, also providing optical pathdifference which is about 1.2 times of the second wavelength for thesecond light flux and further providing optical path difference which isabout 1 time of the third wavelength for the third light flux.

Further, the second basic structure is the optical path differenceproviding structure which emits a 0-th-order diffracted light flux witha larger light amount than any diffracted light fluxes with the otherdiffraction order, when the first light flux passes through the secondbasic structure, which emits a 0-th-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the second light flux passes through the secondbasic structure, and which emits a ±1st order diffracted light flux witha larger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the secondbasic structure. The second basic structure is preferably the opticalpath difference providing structure which emits the first light flux andthe second light flux each of which passes through the second basicstructure with their wavefront substantially being even, and which emitsthe third light flux which passes through the second basic structurewith the wavefront substantially being uneven. Further the second basicstructure is preferably the optical path difference providing structurewhich makes a diffraction angle of the third light flux passing throughthe second basic structure different from the diffraction angles of thefirst light flux and the second light flux each passing through thesecond basic structure. The step amount in the direction of the opticalaxis of the step in the second basic structure is preferably the stepamount providing optical path difference which is about five times ofthe first wavelength for the first light flux, also providing opticalpath difference which is about three times of the second wavelength forthe second light flux and further providing optical path differencewhich is about 2.5 times of the third wavelength for the third lightflux. Further, the shape of the second basic structure is preferably abinary structure shape as shown in FIG. 2( b) as an example.

Further, it is preferable that the second optical path differenceproviding structure is the structure which comprises any one structureselected from the group including at least the first basic structure,the fifth basic structure and the sixth basic structure. It ispreferable that the second optical path difference providing structureis not the structure which overlaps two or more of the first basicstructure, the fifth basic structure and the sixth basic structure. Whenthe second optical path difference providing structure at leastcomprises the first basic structure, the second optical path differenceproviding structure has the same basic structure as the first opticalpath difference providing structure. Therefore, it makes the design workeasy, which is preferable.

The fifth basic structure is the optical path difference providingstructure which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the fifth basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the fifth basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstfifth structure. The step amount in the direction of the optical axis ofthe step in the fifth basic structure is preferably the step amountproviding optical path difference which is about one time of the firstwavelength for the first light flux, also providing optical pathdifference which is about 0.6 times of the second wavelength for thesecond light flux and further providing optical path difference which isabout 0.5 times of the third wavelength for the third light flux.

The sixth basic structure is the optical path difference providingstructure which emits a third-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the sixth basicstructure, which emits a second-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the second light flux passes through the sixthbasic structure, and which emits a second order diffracted light fluxwith a larger light amount than any diffracted light fluxes with theother diffraction order, when the third light flux passes through thesixth basic structure. The step amount in the direction of the opticalaxis of the step in the sixth basic structure is preferably the stepamount providing optical path difference which is about three times ofthe first wavelength for the first light flux, also providing opticalpath difference which is about 1.9 times of the second wavelength forthe second light flux and further providing optical path differencewhich is about 1.6 times of the third wavelength for the third lightflux.

When the objective optical element is a plastic lens, it is preferablethat the first optical path difference providing structure is tripleoverlapped structure in which three kinds of basic structures overlapwith each other. More specifically, it is preferable that the triplebasic structure is formed by overlapping the third basic structure, thefourth basic structure or the seventh basic structure with the firstbasic structure and the second basic structure. More preferably, thetriple overlapped structure is formed by overlapping the third basicstructure with the first basic structure and the second basic structure.

Hereupon, the third basic structure is the optical path differenceproviding structure which emits a tenth-order diffracted light flux witha larger light amount than any diffracted light fluxes with the otherdiffraction order, when the first light flux passes through the thirdbasic structure, which emits a sixth-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the second light flux passes through the thirdbasic structure, and which emits a fifth-order diffracted light fluxwith a larger light amount than any diffracted light fluxes with theother diffraction order, when the third light flux passes through thethird basic structure. The step amount in the direction of the opticalaxis of the step in the third basic structure is preferably the stepamount providing optical path difference which is about ten times of thefirst wavelength for the first light flux, also providing optical pathdifference which is about six times of the second wavelength for thesecond light flux and further providing optical path difference which isabout five times of the third wavelength for the third light flux.Further, the fourth basic structure is the optical path differenceproviding structure which emits a fifth-order diffracted light flux witha larger light amount than any diffracted light fluxes with the otherdiffraction order, when the first light flux passes through the fourthbasic structure, which emits a third-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the second light flux passes through the fourthbasic structure, and which emits third and second order diffracted lightfluxes each with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the third light flux passesthrough the fourth basic structure. It is preferable that the lightamount of the third diffracted light flux is slightly larger than thatof the second order diffracted light flux in the third light flux. Thestep amount in the direction of the optical axis of the step in thefourth basic structure is preferably the step amount providing opticalpath difference which is about five times of the first wavelength forthe first light flux, also providing optical path difference which isabout three times of the second wavelength for the second light flux andfurther providing optical path difference which is about 2.5 times ofthe third wavelength for the third light flux. Further, the seventhbasic structure is the optical path difference providing structure whichemits a second-order diffracted light flux with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe first light flux passes through the fourth basic structure, whichemits a first-order diffracted light flux with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe second light flux passes through the fourth basic structure, andwhich emits first order diffracted light fluxes each with a larger lightamount than any diffracted light fluxes with the other diffractionorder, when the third light flux passes through the fourth basicstructure. The step amount in the direction of the optical axis of thestep in the seventh basic structure is preferably the step amountproviding optical path difference which is about two times of the firstwavelength for the first light flux, also providing optical pathdifference which is about 1.2 times of the second wavelength for thesecond light flux and further providing optical path difference which isabout one time of the third wavelength for the third light flux.Further, the third basic structure, the fourth basic structure and theseventh basic structure have the function so as to make the sphericalaberration under-corrected, when the temperature increases and thewavelength of the first light source, the second light source, and thethird light source increases. Thereby, it can compensate theover-corrected spherical aberration caused with the refractive indexlowering of the plastic at the time of temperature rise, and it allowsto provide an excellent the spherical aberration. Hereupon, the depth ofthe step in the fourth basic structure and the seventh basic structurecan be decreased as compared to that of the third basic structure. It ispreferable that the third basic structure, the fourth basic structureand the seventh basic structure are provided on a basic surface (a basicaspheric surface) which is different from a basic surface (a basicaspheric surface) on which the first basic structure, the second basicstructure, the fifth basic structure and the sixth basic structure areprovided. Further, it is preferable that the third basic structure, thefourth basic structure and the seventh basic structure are provided onthe basic surface which is designed so that the third basic structure,the fourth basic structure and the seventh basic structure does notinfluence as much as possible in the direction of the light fluxincident to the objective optical element. Further it is preferable thatthe third basic structure, the fourth basic structure and the seventhbasic structure are the structure in which the depth of the structure inthe direction of the optical axis becomes deeper as farther from theoptical axis in the direction perpendicular to the optical axis till onepoint, and then in the area farther from the optical axis than the onepoint, the depth of the structure in the direction of the optical axisbecomes shallower as farther from the optical axis in the directionperpendicular to the optical axis.

Further, when the objective optical element is a plastic lens, it ispreferable that the second optical path difference providing structureis formed by overlapping any one of the third basic structure, thefourth basic structure and the seventh basic structure with any one ofthe first basic structure, a fifth basic structure, and a sixth basicstructure. It is more preferable that the second optical path differenceproviding structure is formed by overlapping the fourth basic structurewith the first basic structure.

Further, when the objective optical element is the plastic lens, it ispreferable that the objective optical element comprises the mostperipheral area comprising the third optical path difference providingstructure. In this case, it is preferable that the third optical pathdifference providing structure is the structure at least having any oneof the third basic structure, the fourth basic structure and the seventhbasic structure. Preferably, the third optical difference providingstructure comprises the fourth basic structure.

Accordingly, when the objective optical element is the plastic lens,there is provided one preferable embodiment that the first optical pathdifference providing structure is the triple overlapped structure formedby overlapping three kinds of basic structures, the second optical pathdifference providing structure is the double overlapped structure formedby two kinds of basic structures, and the third optical path differenceproviding structure has only one kind basic structure.

Further, when the objective optical element is a glass lens or a lensformed of athermal resin, it is preferable that the first optical pathdifference providing structure is the structure formed by overlappingonly the first basic structure and the second basic structure.

Further, when the objective optical element is a glass lens or a lensformed of athermal resin, it is preferable that the second optical pathdifference providing structure is the structure formed by overlappingany one of the third structure and the fourth structure with any one ofthe first basic structure, the fifth basic structure or the sixth basicstructure. Preferably, it is the structure formed by overlapping thefirst basic structure and the fourth basic structure.

Further, when the objective optical element is a glass lens or a lensformed of athermal resin, it is preferable that the objective opticalelement comprises the most peripheral area which is the refractivesurface.

It is preferable that the first optical path difference providingstructure is a ring-shaped structure comprising plural ring-shaped zoneswith steps which are arranged concentrically. When the objective opticalelement is a plastic lens, it is preferable that the steps in the firstoptical path difference providing structure have at least two kinds ofstep amounts selected from dA, dB, dC, and dD represented by thefollowing expressions.

0.9×(15λB/(n−1)−2λB′/(n′−1))<dA(μm)

<1.5×(15λB/(n−1)−2λB′/(n′−1))  (17)

0.9×(5λB/(n−1)+2λB′/(n′−1))<dB(μm)

<1.5×((5λB/(n−1)+2λB′/(n′−1))  (18)

0.9×(5λB/(n−1)<dC(μm)<1.5×5λB/(n−1)  (19)

0.9×(5λB/(n−1)−2λB′/(n′−1))<dD(μm)

<1.5×(5λB/(n−1)−2λB′/(n′−1))  (20)

Where, dA preferably satisfies the following expression (17′).

0.95×(15λB/(n−1)−2λB′/(n′−1))<dA(μm)

<1.4×(15λB/(n−1)−2λB′/(n′−1))  (17″)

Further, dA more preferably satisfies the following expression (17″).

1.0×(15λB/(n−1)−2λB′/(n′−1))≦dA(μm)

<1.3×(15λB/(n−1)−2λB′/(n′−1))  (17″)

Where, dB preferably satisfies the following expression (18′).

0.95×(5λB/(n−1)+2λB′/(n′−1))<dB(μm)

<1.4×((5λB/(n−1)+2λB′/(n′−1))  (18′)

Further, dB more preferably satisfies the following expression (18″).

1.0×(5λB/(n−1)+2λB′/(n′−1))≦dB(μm)

<1.3×((5λB/(n−1)+2λB′/(n′−1))  (18″)

Where, dC preferably satisfies the following expression (19′).

0.95×5λB/(n−1)<dC(μm)<1.4×5λB/(n−1)  (19′)

Further, dC more preferably satisfies the following expression (19″).

1.0×5λB/(n−1)<dC(μm)<1.3×5λB/(n−1)  (19″)

Where, dD preferably satisfies the following expression (20′).

0.95×(5λB/(n−1)−2λB′/(n′−1))<dD(μm)

<1.4×(5λB/(n−1)−2λB′/(n′−1))  (20′)

Further, dD more preferably satisfies the following expression (20″).

1.0×(5λB/(n−1)−2λB′/(n′−1))<dD(μm)

<1.3×(5λB/(n−1)−2λB′/(n′−1))  (20″)

Where, λB represents a design wavelength (μm); λB′ represents anarbitral value which is 0.390 μm or more, and is 0.410 μm or less; nrepresents a refractive index of the objective optical element for thedesign wavelength λB; and n′ represents a refractive index of theobjective optical element for a wavelength λB′.

Further, λB may represent the wavelength (μm) of the first light sourcearranged in the optical pickup apparatus, that is using wavelength, ifit is difficult to perceive the design wavelength; and λB′ preferablyrepresents an arbitral value which is 0.390 μm or more, and is 0.405 μmor less. More preferably, λB′ represents an arbital value which is 0.390μm or more, and is 0.400 μm or less.

In the specification, the step amount means a length of the step in theoptical path difference providing structure in the direction of theoptical axis. For example, when the optical path difference providingstructure is the structure shown in FIG. 12, examples of the step amountare each length of d1, d2, d3, and d4. “The steps in the first opticalpath difference providing structure have at least two kinds of stepamounts selected from dA, dB, dC, and dD” means that at least one step xin the entire of the steps in the first optical path differenceproviding structure has the step amount satisfying any one of dA, dB,dC, and dD, and the other step y has the step amount satisfying any oneof dA, dB, dC, and dD, which differs from the step amount of the step x.

It is preferable that there is no step having the step amount other thandA, dB, dC, and dD in the first optical path difference providingstructure. It is further preferable that the step amount is notexcessively large, from the viewpoint of easy manufacture of the die forthe optical path difference providing structure and from the viewpointof excellent transfer of the die. Therefore, it is more preferable thatthe entire of steps in the first optical path difference providingstructure are composed of the steps having the step amounts satisfyingonly dC and dD.

When designing an optical element according to the present invention, itcan be considered to design it with the following method. First, thebasic structure which is an optical path difference providing structurecomprising a ring-shaped structure, is designed. Next, another basicstructure is designed and the another basic structure comprisies aring-shaped structure which emits different order diffracted light fluxwith the maximum light amount for some wavelength from the diffractedlight flux with the maximum light amount emitted from the former basicstructure. Then, the first optical path difference providing structureor the second optical path difference providing structure can bedesigned by overlapping these two basic structures (they may be threebasic structures) with each other. When designing the optical pathdifference providing structure using this method, there is a possibilitythat a ring-shaped zone with small pitch width appears. For example, theoptical path difference providing structure shown in FIG. 14( c) can beobtained by overlapping the basic structure shown in FIG. 14( a) and thebasic structure shown in FIG. 14( b). However, there appears a smallring-shaped zone with a small pitch width which is shown as Wa in FIG.14( c) in the obtained structure. Here, the pitch width means a width ofthe ring-shaped zone in the direction perpendicular to the optical axisof the optical element. For example, when the optical path differenceproviding structure is a structure shown in FIG. 12, examples of thepitch width are each length of w1, w2, w3, and w4. When the optical pathdifference providing structure is a structure shown in FIG. 13, examplesof the pitch width are each length of w5, w6, w7, w8, and w9.

The inventors of the present invention found as a result of earneststudy that when the ring-shaped zone has width Wa of 5 μm or less, thering-shaped zone can be removed or filled without giving bad effect tothe optical characteristics. Namely, in the structure shown in FIG. 14(c), the optical characteristics is not substantially affected, even whenthe small pitch width in which Wa is 5 μm or less is removed.

Further, it is preferable that the pitch width of the step is notexcessively small, from the viewpoint of manufacturing the die easilyand transferring the die in good condition. Therefore, when anring-shaped zone with the pitch width of 5 μm or less appears in theprimitive optical path difference providing structure designed byoverlapping plural basic structures, it is preferable that thering-shaped zone with the pitch width of 5 μm or less is removed toobtain the final version of the optical path difference providingstructure. When the ring-shaped zone with the pitch width of 5 μm orless is the protruded shape, the ring-shaped zone can be removed bycutting out. When the ring-shaped zone with the pitch width of 5 μm orless is the depressed shape, the ring-shaped zone can be removed byfilling up.

Accordingly, at least the first optical path difference providingstructure are composed of the ring-shaped zones whole of which havepitch width of 5 μm or more. Preferably, the second optical pathdifference providing structure and the third optical path differenceproviding structure also are composed of the ring-shaped zones whole ofwhich have pitch width of 5 μm or more.

As described above, it is preferable that the step amount is notexcessively large. The inventors of the present invention found thefollowings as a result of earnest study. When the primitive optical pathdifference providing structure obtained by overlapping plural basicstructures has one ring-shaped zone with a step amount being larger thanthe reference value, the excessively large step amount can be reducedwithout bad effect to the optical characteristics by making the stepamount of the ring-shaped zone small by 10·λB/(n−1) μm. Although anarbitral value is set for the reference value, the reference value ispreferably set to 10·λB/(n−1) μm.

From a viewpoint that fewer thin ring-shaped zones are preferable, it ispreferable that the entire of the ring shaped zones of the first opticalpath difference providing structure satisfies that the value of (stepamount/pitch width) is preferably 1 or less, and more preferably is 0.8or less. Further more preferably, the entire of the ring shaped zones ofall of the optical path difference providing structures satisfies thatthe value of (step amount/pitch width) is preferably 1 or less, and mostpreferably is 0.8 or less.

NA1 represents the image side numerical aperture of the objectiveoptical element, necessary for reproducing and/or recording informationfor the first optical disk. NA2 (NA1≧NA2) represents that the image sidenumerical aperture of the objective optical element necessary forreproducing and/or recording for the information to the second opticaldisk. NA3 (NA2>NA3) represents that the image side numerical aperture ofthe objective optical element necessary for reproducing and/or recordinginformation for the third optical disk. It is preferable that NA1 is oneof: 0.8 or more, and 0.9 or less; and 0.55 or more, and 0.7 or less.Specifically, preferable NA1 is 0.85. It is preferable that NA2 is 0.55or more, and is 0.7 or less. Specifically, preferable NA2 is 0.60.Further, it is preferable that NA3 is 0.4 or more, and is 0.55 or less.Specifically, preferable NA3 is 0.45 or 0.53.

It is preferable that the border of the central area and the peripheralarea in the objective optical element is formed in a portioncorresponding to the range being 0.9·NA3 or more and being 1.2·NA3 orless (more preferably, 0.95·NA3 or more, and 1.15·NA3 or less) for thethird light flux. More preferably, the border of the central area andthe peripheral area of the objective optical element is formed in aportion corresponding to NA3. Further, it is preferable that the borderof the peripheral area and the most peripheral area of the objectiveoptical element is formed in a portion corresponding to the range being0.9·NA2 or more, and being 1.2·NA2 or less (more preferably, being0.95·NA2 or more, and being 1.15·NA2 or less) for the second light flux.More preferably, the border of the peripheral area and the mostperipheral area of the objective optical element is formed in a portioncorresponding to NA2. It is preferable that the border of the outside ofthe most peripheral area of the objective optical element is formed in aportion corresponding to the range being than 0.9·NA1 or more, and being1.2·NA1 or less (more preferably, being 0.95·NA1 or more, and being1.15·NA1 or less) for the first light flux. More preferably, the borderof the outside of the most peripheral area of the objective opticalelement is formed in a portion corresponding to NA1.

When the third light flux passing through the objective optical elementis converged on the information recording surface of the third opticaldisk, it is preferable that the spherical aberration has at least onediscontinuous portion. In that case, it is preferable that thediscontinuous portion exists in the range being 0.9·NA3 or more, andbeing 1.2·NA3 or less (more preferably, being 0.95·NA3 or more, andbeing 1.15·NA3 or less) for the third light flux. Further, also when thesecond light flux passing through the objective optical element isconverged on the information recording surface of the second opticaldisk, it is preferable that the spherical aberration has at least onediscontinuous portion. In that case, it is preferable that thediscontinuous portion exists in the range being 0.9·NA2 or more, andbeing 1.2·NA2 or less (more preferably, being 0.95·NA2 or more, andbeing 1.1·NA2 or less) for the second light flux.

Further, when the spherical aberration is continuous and does not havethe discontinuous portion, and when the third light flux passing throughthe objective optical element is converged on the information recordingsurface of the third optical disk, it is preferable that the absolutevalue of the vertical spherical aberration is 0.03 μm or more in NA2,and the absolute value of the vertical spherical aberration is 0.02 μmor less in NA3. More preferably, the absolute value of the verticalspherical aberration is 0.08 μm or more in NA2, and the absolute valueof the vertical spherical aberration is 0.01 μm or less in NA3. Further,when the second light flux passing through the objective optical elementis converged on the information recording surface of the second opticaldisk, it is preferable that the absolute value of the vertical sphericalaberration is 0.03 μm or more in NA1, and the absolute value of thevertical spherical aberration is 0.005 μm or less in NA2.

Further, because the diffraction efficiency depends on the depth (stepamount) of the ring shaped zone in the diffractive structure, thediffraction efficiency of the central area for each wavelength can beappropriately set corresponding to the use of the optical pickupapparatus. For example, in the case of the optical pickup apparatus forrecording and reproducing information on the first optical disk, andonly for reproducing information on the second and the third opticaldisks, it is preferable that the diffraction efficiency of the centralarea and/or the peripheral area is defined with considering primarilythe diffraction efficiency for the first light flux. On the other hand,in the case of the optical pickup apparatus only for reproducinginformation on the fist optical disks and for recording and reproducinginformation on the second and third optical disks, it is preferable thatthe diffraction efficiency of the central area is defined withconsidering primarily the diffraction efficiency for the second andthird light fluxes and the peripheral area is defined with consideringprimarily the diffraction efficiency for the second light flux.

In any case, when the following conditional expression (11) issatisfied, the diffraction efficiency of the first light flux calculatedby the area weighted mean can be secured high.

η11≦η21  (11)

Where, η11 expresses the diffraction efficiency of the first light fluxin the central area, η21 expresses the diffraction efficiency of thefirst light flux in the peripheral area. Hereupon, when the diffractionefficiency of the central area is defined with considering primarily thelight fluxes with the second and the third wavelengths, the diffractionefficiency of the first light flux of the central area is decreased.However, in the case where the numerical aperture of the first opticaldisk is larger than the numerical aperture of the third optical disk,when considered on the whole effective diameter of the first light flux,the diffraction efficiency decrease of the central area does not give somuch large influence.

Hereupon, the diffraction efficiency in the present specification can bedefined as follows.

-   (1) The transmittance of an objective optical element excluding the    first and the second optical path difference providing structures is    measured with being divided into the central area and the peripheral    area. The objective optical element without the first and the second    optical path difference providing structures has a focal length, a    lens thickness, a numerical aperture, a material which are the same    to the objective optical element having the first and the second    optical path difference providing structures. In this case, the    transmittance of the central area is measured with the light flux    which enters into the peripheral area being shielded, and the    transmittance of the peripheral area is measured with the light flux    which enters into the central area being shielded.-   (2) The transmittance of the objective optical element having the    first and the second optical path difference providing structures is    measured with being divided into the central area and the peripheral    area.-   (3) The diffraction efficiencies of both areas are obtained by    dividing the results of (2) is divided by the respective results of    (1).

Further, the light utilization efficiency of any two light fluxes of thefirst light flux to the third light flux may be 80% or more, and thelight utilization efficiency of the remaining one light flux may be 30%or more, and 80% or less. The light utilization efficiency of theremaining one light flux may also be 40% or more, and 70% or less. Inthis case, it is preferable that the light utilization efficiency of thethird light flux is 30% or more, and 80% or less (or 40% or more, and70% or less) is.

Hereupon, the light utilization efficiency described in thisspecification is calculated as follows. It is defined that the objectivelens which includes the first optical path difference providingstructure and the second optical path difference providing structure(which may further include the third optical path difference providingstructure) forms an air disk whose light amount is A, on the informationrecording surface. It is further defined that the objective lens whichis formed of the same material to the former objective lens and has thesame focal length, axial thickness, numerical aperture, and wavefrontaberration to the former objective lens and which does not includes thefirst optical path difference providing structure, the second opticalpath difference providing structure, and the third optical pathdifference providing structure, forms an air disk whose light amount B,on the information recording surface. The light utilization efficiencydescribed in this specification is obtained by calculating A/B. Where,the above described airy disk is the circle with a radius r′ formedaround the optical axis of the light converged spot. The radius r′ isexpressed by r′=0.61·λ/NA.

Further, it is difficult that the tracking characteristic particularlyin the third optical disk is maintained in the good condition when thereis provided the light amount difference being 0% or more, and 20% orless between the diffracted light flux with a diffraction order havingthe largest light amount and the diffracted light flux with anotherdiffraction order having the second largest light amount in the thirdlight flux which has passed through the first optical path differenceproviding structure, that is, the difference between the diffractedlight flux forming the first best focus, and the diffracted light fluxforming the second best focus. However, the embodiment of the presentinvention can provide a excellent tracking characteristics under theabove described condition.

Each of the first light flux, the second light flux and the third lightflux may enters into the objective optical element as a collimated lightflux, or may also enter into the objective optical element as adivergent light flux or a convergent light flux. Preferably, themagnification m1 of the objective optical element when the first lightflux enters into the objective optical element satisfies the followingexpression (2).

−0.02<m1<0.02  (2)

On the one hand, when the first light flux is made incident on theobjective optical element as the divergent light, it is preferable thatthe magnification m1 of the incident light flux of the first light fluxon the objective optical element satisfies the following expression(2′).

−0.10<m1<0.00  (2′)

Further, when the second light flux enters into the objective opticalelement as the collimated light flux or substantially collimated lightflux, it is preferable that the magnification m2 of the objectiveoptical element when the second light flux enters into the objectiveoptical element satisfies the following expression (3).

−0.02<m2<0.02  (3)

On the one hand, when the second light flux is made incident on theobjective optical element as the divergent light, it is preferable thatthe magnification m2 of the incident light flux of the second light fluxon the objective optical element satisfies the following expression(3′).

−0.10<m2<0.00  (3′)

Further, when the third light flux enters into the objective opticalelement as a collimated light flux or substantially collimated lightflux, it is preferable that the magnification m3 of the objectiveoptical element when the third light flux enters into the objectiveoptical element satisfies the following expression (4). When the thirdlight flux is a collimated light flux, a problem is easily caused in thetracking operation. However, even when the third light flux is acollimated light flux, the embodiment of the present invention canobtain the good tracking characteristics, and information can beadequately recorded and/or reproduced for the three different opticaldisks.

−0.02<m3<0.02  (4)

On the one hand, when the third light flux enters into the objectiveoptical element as a divergent light flux, it is preferable that themagnification m3 of the objective optical element when the third lightflux enters into the objective optical element satisfies the followingexpression (5).

−0.10<m3<0.00  (5)

Further, when the objective optical element is a single plastic lens, itis preferable that the temperature characteristic makes good even ifsome wavelength characteristics is sacrificed. Particularly, it ispreferable to maintain a good balance between the wavelength andtemperature characteristics. It is more preferable to maintain thetemperature characteristic when recording and/or reproducing informationfor the first optical disk, is made good. In order to satisfy such acharacteristic, it is preferable to satisfy the following conditionalexpressions (12) and (13).

+0.00045(WFEλrms/(° C.·mm))≦δSAT1/f

≦+0.0027(WFEλrms/(° C.·mm))  (12)

−0.045(WFEλrms/(nm·mm))≦δSAX/f

≦−0.0045(WFEλrms/(nm·mm))  (13)

Where, δSAT1 represents δSA3/δT of the objective optical element for ausing light wavelength (In this case, it is defined that the wavelengthdoes not changes with the temperature change), in case that informationis recorded and/or reproduced for the first optical disk. The usingwavelength means a wavelength of the first light flux emitted from thefirst light source arranged in the optical pickup apparatus comprisingthe objective optical element. The preferable using wavelength is in therange being 400 nm or more, and being 415 nm or less, and is awavelength which enables to record and/or reproduce information for thefirst optical disk. When the using wavelength is difficult to be set asabove, the using wavelength may be set to 405 nm and δSAT1 of theobjective optical element and δSAT2 and δSAT3 of the objective opticalelement which are described later, may be obtained. In other words,δSAT1 is a ratio of a change in a third spherical aberration of theobjective optical element with a temperature change (temperaturecharacteristics) for a using light wavelength which does not change withtemperature change, in case that information is recorded and/orreproduced for the first optical disk. Here, WFE means that the thirdspherical aberration is represented by using the wavelength aberration.δSAλ represents δSA3/δλ when information is recorded and/or reproducedfor the first optical disc by the using wavelength under the conditionthat the ambient temperature is constant. In other words, δSAλrepresents a change in the third spherical aberration of the objectiveoptical element with the wavelength change (wavelength characteristics)when information is recorded and/or reproduced for the first opticaldisk by the using wavelength under the condition that the ambienttemperature is constant. Here, it is preferable that the ambienttemperature is a room temperature. The room temperature indicates 10° C.or more, and 40° C. or less, and preferably indicates 25° C. In aboveexpressions, f is a focal length of the objective optical element forthe using wavelength of the first light flux, which is preferably 405nm.

More preferably, the following conditional expression (12′) issatisfied.

+0.00091(WFEλrms/(° C.·mm))≦δSAT1/f

≦+0.0018(WFEλrms/(° C.·mm))  (12′)

Further preferably, the following conditional expression (12″) issatisfied.

+0.0013(WFEλrms/(° C.·mm))≦δSAT1/f

≦+0.0016(WFEλrms/(° C.·mm))  (12″)

Preferably, the following conditional expression (12′″) is satisfied.

|δSAT1|≦0.002(WFEλrms/° C.)  (12′″)

Preferably, the following conditional expression (13′) is satisfied,more preferably, the following conditional expression (13″) issatisfied.

−0.032(WFEλrms/(nm·mm))≦≦δSAλ/f

≦−0.0091(WFEλrms/(nm·mm))  (13′)

−0.015(WFEλrms/(nm·mm))≦δSAλ/f

≦−0.011(WFEλrms/(nm·mm))  (13″)

Preferably, the following conditional expression (13′″) is satisfied,and more preferably, the following conditional expression (13″″) issatisfied.

0.01(WFEλrms/nm)≦|δSAλ|

<0.1(WFEλrms/nm)  (13′″)

0.02(WFEλrms/nm)≦|δSAλ|

<0.1(WFEλrms/nm)  (13″″)

Describing furthermore, it is preferable that the objective opticalelement has the wavelength dependency of the spherical aberration so asto correct the change in the spherical aberration due to the refractiveindex change of the objective optical element caused with thetemperature change by the change of the first wavelength due to thetemperature change. Preferably, the following conditional expression(14) is satisfied.

0(WFEλrms/(° C.·mm))≦δSAT2/f

≦+0.00136(WFEλrms/(° C.·mm))  (14)

Where, δSAT2 represents δSA3/δT of the objective optical element for ausing light wavelength (preferably 405 nm) which changes by 0.05 nm/° C.with a temperature change, in case that information is recorded and/orreproduced for the first optical disk. In other words, δSAT2 is a ratioof a change in a third spherical aberration of the objective opticalelement with a temperature change for a using light wavelength(preferably 405 nm) which changes by 0.05 nm/° C. with a temperaturechange in case that information is recorded and/or reproduced for thefirst optical disk.

More preferably, the following conditional expression (14′) issatisfied.

0(WFEλrms/(° C.·mm))≦δSAT2/f

≦+0.00093(WFEλrms/(° C.·mm))  (14′)

Further preferably, the following conditional expression (14″) issatisfied.

+0.0007(WFEλrms/(° C.·mm))≦δSAT2/f

≦+0.0009(WFEλrms/(° C.·mm))  (14″)

Further, when the light converging optical system of the optical pickupapparatus comprises a coupling lens such as a collimator lens, and whenthe coupling lens is a plastic lens, it preferably satisfies thefollowing conditional expression (15).

0(WFEλrms/(° C.·mm))≦δSAT3/f

≦+0.00091(WFEλrms/(° C.·mm))  (15)

Where, δSAT3 represents δSA3/δT of a total optical system including thecoupling lens and the objective optical element for a using lightwavelength (preferably 405 nm) which changes by 0.05 nm/° C. with atemperature change, in case that information is recorded and/orreproduced for the first optical disk. In other words, δSAT3 is a ratioof a change in a third spherical aberration of a total optical systemincluding the coupling lens and the objective optical element with atemperature change (temperature characteristics) for a using lightwavelength (preferably 405 nm) which changes by 0.05 nm/° C. with thetemperature change, in case that information is recorded and/orreproduced for the first optical disk.

More preferably, the following conditional expression (15′) issatisfied.

0(WFEλrms/(° C.·mm))≦δSAT3/f

≦+0.00045(WFEλrms/(° C.·mm))  (15′)

More preferably, the following conditional expression (15″) issatisfied.

+0.00005(WFEλrms/(° C.·mm))≦δSAT3/f

≦+0.0003(WFEλrms/(° C.·mm))  (15″)

As described above, when the wavelength characteristic is not made somuch good, the objective optical element whose temperaturecharacteristic is excellent can be obtained. As for the wavelengthcharacteristics, the light source whose oscillation wavelength isappropriate for the objective optical element is selected withconsidering the fluctuation of the oscillation wavelength of the lightsource, that is, the bad influence came from the fluctuation of thewavelength can be suppressed to some degree by selecting the lightsource whose oscillation wavelength is close to the referencewavelength. Accordingly, a low cost and simple optical pickup apparatuscan be provided by combining the objective optical element as describedabove and the light source whose oscillation wavelength is severelyselected. Further, there can be previously prepared the different kindsof the above described objective optical elements whose designwavelengths are different from each other. Therefore, it allows toprovide a low cost and simple optical pickup apparatus by combining thelight source and the objective optical element whose designed wavelengthis adequate corresponding to the fluctuation of the oscillationwavelength of the light source. More specifically, it is preferable thatthe manufacturing method of the optical pickup apparatus includes: thestep of dividing the objective optical elements whose temperaturecharacteristic is excellent as described above, into a plurality ofgroups corresponding to the specification of the optical path differenceproviding structure; the step of selecting any one group of theobjective optical element corresponding to the oscillationcharacteristics of the light source to be used; and the step ofcombining the light source and the selected objective optical element.

For example, there is provided the objective optical elements which aredivided into groups including: the group of the objective opticalelements having an optical path difference providing structureappropriate for the light source whose oscillation wavelength isdeviated to plus side to the reference wavelength; a group of theobjective optical elements having an optical path difference providingstructure appropriate for the light source whose oscillation wavelengthis close to the reference wavelength; and a group of the objectiveoptical elements having the optical path difference providing structureappropriate for the light source whose oscillation wavelength isdeviated to minus side to the reference wavelength. Then, theoscillation wavelength of the using light source is measured. Theobjective optical element lens is selected from the group mostappropriate for the measured light source and is combined with themeasured light source. These steps allows to provide the optical pickupapparatus which can adequately record and/or reproduce information, evenwhen the temperature changes. Further, it is preferable that, thedifferent identification marks corresponding to these groups areprovided to the objective optical elements. In above description, “thespecification of the optical path difference providing structure isdifferent” means, for example, that the design wavelength of thediffractive structure is made different. However, it is to be understoodthat various changes and modifications will be apparent to those skilledin the art. Further, “oscillation characteristics” includes the measuredvalue and fluctuation of the oscillation wavelength.

Further, when the manufacturing method of the above-described opticalpickup apparatus is applied, and when the delivery lot of themanufactured optical pickup apparatus is inspected, it is preferablethat the following conditional expression (16) is satisfied.

σSA3≦σλ·δSA3λ_(a11)  (16)

Where, σSA3 (λrms) expresses the standard deviation of the third orderspherical aberration of the light converging optical systems (from thelight source to the information recording surface) each including theobjective optical element of the optical pickup apparatus for thewavelength of the light source oscillated at the reference temperatureand reference output. Further, σλ (nm) expresses the standard deviationof the oscillation wavelength at the reference temperature and referenceoutput of the light sources included in the delivery lot of the opticalpickup apparatus. Further, δSA3λ_(a11) (λrms/nm) expresses thewavelength dependency of the third spherical aberration of the lightconverging optical systems (from the light source to the informationrecording surface) each including the objective optical element includedin the delivery lot of the optical pickup apparatus.

Herein, the description will be returned to the explanation of theobjective optical element. It is preferable that the objective opticalelement satisfies the above-described conditional expressions (12) to(15) by including the temperature characteristics correction structure.For example, in the case where the first optical path differenceproviding structure is the structure comprising at least any onestructure selected from a group including at least the third basicstructure, the fourth basic structure and the seventh basic structure,the above described conditional expressions (12) to (15) can be realizedwithout designing the complicated optical element. Further, when thesecond optical path difference providing structure is the structurecomprising any one structure selected from a group including at leastthe third basic structure, fourth basic structure and the seventhstructure, the above described conditional expressions (12), (12′),(13), (13′), (13″), (14), (14′), (15), and (15′) can be realized withoutdesigning the complicated optical element. Further, when the objectiveoptical element comprises the most peripheral area comprising the thirdoptical path difference providing structure around the peripheral area,and the third optical path difference providing structure is thestructure comprising any one structure selected from a group includingat least the third basic structure, the fourth basic structure and theseventh basic structure, the above-described conditional expressions(12), (12′), (13), (13′), (13″), (14), (14′), (15), and (15′) can berealized without designing the complicated optical element.

Further, in the case where the image side numerical aperture (NA) of theobjective optical element to the first light flux is 0.8 or more, and0.9 or less, the effects when the above-described conditionalexpressions (12), (12′), (13), (13′), (13″), (14), (14′), (15), (15′)are satisfied, are more conspicuous.

Further, it is preferable that the working distance (WD) of theobjective optical element when the third optical disk is used, is 0.20mm or more, and is 1.5 mm or less. The working distance of the objectiveoptical element when the third optical disk is preferably 0.3 mm ormore, and 1.0 mm or less. Next, it is preferable that the workingdistance (WD) of the objective optical element when the second opticaldisk is used, is 0.4 mm or more, and is 0.7 mm or less. The workingdistance is preferably that the working distance (WD) of the objectiveoptical element when the first optical disk is used, is 0.4 mm or more,and is 0.9 mm or less (when t1<t2, it is preferably 0.6 mm or more, andis 0.9 mm or less).

It is preferable that the entrance pupil diameter of the objectiveoptical element is 2.8 mm or more, and is 4.5 mm or less when the firstoptical disk is used.

The optical information recording and reproducing apparatus according tothe present invention, has the optical disk drive apparatus having theabove described optical pickup apparatus.

Herein, the optical disk drive apparatus installed in the opticalinformation recording and reproducing apparatus will be described. Thereis provided the optical disk drive apparatus employing a system oftaking out only a tray which can hold an optical disk with the opticaldisk being, from the main body of the optical information recording andreproducing apparatus in which optical pickup apparatus is housed; and asystem of taking out the main body of the optical disk drive apparatusin which the optical pickup apparatus is housed.

The optical information recording and reproducing apparatus using eachof the above described systems, is generally provided with the followingcomponent members: an optical pickup apparatus housed in a housing; adrive source of the optical pickup apparatus such as seek-motor by whichthe optical pickup apparatus is moved toward the inner periphery orouter periphery of the optical disk for each housing; traveling meanshaving a guide rail for guiding the optical pickup apparatus toward theinner periphery or outer periphery of the optical disk; and a spindlemotor for rotation driving of the optical disk. However, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art.

The optical information recording and reproducing apparatus employingthe former system is provide with, other than these component members, atray which can hold the optical disk with the optical disk being mountedthereon, and a loading mechanism for slidably moving the tray. Theoptical information recording and reproducing apparatus employing thelatter system does not include the tray and loading mechanism, and it ispreferable that each component member is provided in the drawercorresponding to chassis which can be taken out outside.

The embodiment of the present invention allows that one pickup lens toproperly record and/or reproduce information for three different disks(for example, a high density optical disk using a blue-violet laserlight source, DVD, and CD) with employing a simple and low-costconfiguration. Further, the above embodiment allows to provide anoptical pickup apparatus, an objective optical element, and an opticalinformation recording reproducing apparatus which can maintain accuracyof the tracking operation for all three different optical disks, evenwhen using an infinite optical system. The above embodiment furtherallows to provide an optical pickup apparatus, an objective opticalelement, and an optical information recording reproducing apparatuswhich properly record and/or reproduce information for three differentdisks with using a single objective lens. The above embodiment furtherallow to provide an optical pickup apparatus, an objective opticalelement, and an optical information recording reproducing apparatuswhich provide excellent temperature characteristics and properly recordand/or reproduce information for three different disks with using asingle objective lens, even when employing a plastic lens.

EXAMPLE

Referring to the drawings, the embodiment of the present invention willbe described below. FIG. 3 is a view schematically showing opticalpickup apparatus PU1 of the present embodiment capable of recordingand/or reproducing information adequately for BD, DVD and CD which aredifferent optical disks. The optical pickup apparatus PU1 can be mountedin the optical information recording and reproducing apparatus. Herein,the first optical disk is BD, the second optical disk is DVD, and thethird optical disk is CD. Hereupon, the present invention is not limitedto the present embodiment.

The optical pickup apparatus PU1 comprises objective optical elementOBJ; aperture ST; collimator lens CL; polarizing dichroic prism PPS;first semiconductor laser LD1 (the first light source) which emits alaser light flux with a wavelength of 405 nm (the firs light flux) whenrecording/reproducing information for BD; and first light receivingelement PD1 which receives the reflection light from informationrecording surface RL1 of BD; and laser module LM.

Further, the laser module LM comprises second semiconductor laser EP1(the second light source) which emits the laser light flux with awavelength of 658 nm (the second light flux) when recording and/orreproducing information for DVD; third semiconductor laser EP2 (thethird light source) emitting the laser light flux with a wavelength of785 nm (the third light flux) when recording and/or reproducinginformation for CD; second light receiving element DS1 which receivesthe reflection light flux from the information recording surface RL2 ofDVD; the third light receiving element DS2 which receives the reflectionlight flux from the information recording surface RL3 of CD; and a prismPS.

As shown in FIG. 1 and FIG. 4, objective optical element OBJ includescentral area CN including the optical axis; peripheral area MD arrangedaround the central area; and most peripheral area OT further arrangedaround the peripheral area, which are formed concentrically around theoptical axis in the aspheric optical surface of the light source side ofthe objective optical element. Hereupon, the area ratio of the centralarea, peripheral area, most peripheral area shown in FIG. 1 and FIG. 4is not expressed exactly.

Blue-violet semiconductor laser diode LD1 emits a first light flux(λ1=405 nm) which is a divergent light flux. The divergent light fluxpasses through polarizing dichroic prism PPS, and is converted into acollimated light flux by collimator lens CL. The collimated light fluxis converted from straight line polarized light into circular polarizedlight by the ¼ wavelength plate which is not shown. The diameter of theconverted light flux is regulated by the aperture ST, and objectiveoptical element OBJ forms the regulated light flux into a spot oninformation recording surface RL1 of BD through the protective substratewith thickness of 0.0875 mm.

The light flux on information recording surface RL1 is reflected andmodulated by the information pit on the information recording surfaceRL1. The reflected light flux passes through objective optical elementOBJ, aperture ST again, and is converted from circular polarized lightinto straight line polarized light by the ¼ wavelength plate which isnot shown. Then, collimator lens CL convert the light flux into aconvergent light flux. The convergent light flux passes throughpolarizing dichroic prism PPS and is converged on the light receivingsurface of the first light receiving element PD1. Then, informationrecorded in BD can be read based on the output signal of the first lightreceiving element PD1, by focusing or tracking objective optical elementOBJ using biaxial actuator AC.

Red semiconductor laser EP1 emits a second light flux (λ2=658 nm) whichis a divergent light flux. The divergent light flux is reflected by theprism PS and is further reflected by polarizing dichroic prism PPS.Collimator lens CL collimate the reflected light flux and the collimatedlight flux is converted from straight line polarized light into circularpolarized light by the ¼ wavelength plate which is not shown. Theconverted light flux enters into objective optical element OBJ. Herein,the incident light flux is converged by the central area and theperipheral area of the objective optical element OBJ (the light fluxpassing through the most peripheral area is made into a flare, and formsthe peripheral spot portion). The converged light flux becomes a spot oninformation recording surface RL2 of DVD through the protectivesubstrate PL2 with a thickness of 0.6 mm, and forms the central spotportion.

The light flux on information recording surface RL2 is reflected andmodulated by the information pit on the information recording surfaceRL2. The reflection light flux passes through objective optical elementOBJ and aperture ST again, and is converted from circular polarizedlight into straight line polarized light by the ¼ wavelength plate whichis not shown. Then, Collimator lens CL converts the light flux into aconvergent light flux, the convergent light flux is reflected by thepolarizing dichroic prism PPS, then, is reflected two times in theprism, and converged on the second light receiving element DS1. Then,the information recorded in DVD can be read by using the output signalof the second light receiving element DS1.

Infrared semiconductor laser EP2 emits the third light flux (λ3=785 nm)which is a divergent light flux. The divergent light flux is reflectedby prism PS, and further reflected by polarizing dichroic prism PPS.Collimator lens CL collimate the reflected light flux and the collimatedlight flux is converted from straight line polarized light into circularpolarized light by the ¼ wavelength plate which is not shown. Theconverted light flux enters into objective optical element OBJ. Herein,the incident light flux is converged by the central area of theobjective optical element OBJ (the light flux passing through theperipheral area and the most peripheral area is made into a flare, andforms the peripheral spot portion). The converged light flux becomes aspot on information recording surface RL3 of CD through the protectivesubstrate PL3 with thickness of 1.2 mm, and forms the central spotportion.

The light flux on information recording surface RL3 is reflected andmodulated by the information pit on the information recording surfaceRL3. The reflection light flux passes through objective optical elementOBJ and aperture ST again, and is converted from circular polarizedlight into straight line polarized light by the ¼ wavelength plate whichis not shown. Then, collimator lens CL converts the light flux into aconvergent light flux, the convergent light flux is reflected by thepolarizing dichroic prism PPS, then, is further reflected two times inthe prism. The reflected light flux is converged on the third lightreceiving element DS2. Then, information recorded in CD can be read byusing output signal of the third light receiving element DS2.

When the first light flux emitted from the blue-violet semiconductorlaser LD1 enters into objective optical element OBJ as a collimatedlight flux, the first optical path difference providing structure of thecentral area, the second optical path difference providing structure ofthe peripheral area and the most peripheral area adequately corrects thespherical aberration of the first light flux. Therefore, information canbe recorded and/or reproduced adequately for BD having a protectivesubstrate with a thickness t1. Further, when the second light fluxemitted from the red semiconductor laser EP1 enters into objectiveoptical element OBJ as a collimated light flux, the first optical pathdifference providing structure of the central area, the second opticalpath difference providing structure of the peripheral area adequatelycorrects the spherical aberration of the second light flux generated dueto the thickness difference between the protective substrates of BD andDVD and the wavelength difference between the first light flux and thesecond light flux, and the most peripheral area makes the second lightflux the flare on the information recording surface of DVD. Therefore,information can be recorded and/or reproduced adequately for DVD havinga protective substrate with thickness of t2. Further, when the thirdlight flux emitted from the infrared semiconductor laser EP2 enters intoobjective optical element OBJ as the parallel light flux, the firstoptical path difference providing structure of the central areaadequately corrects the spherical aberration of the third light fluxgenerated due to the thickness difference between the protectivesubstrates of BD and CD and the wavelength difference between the firstlight flux and the third light flux, and the second optical pathdifference providing structure of the peripheral area and the mostperipheral area make the third light flux the flare on the informationrecording surface of CD. Therefore, information can be recorded and/orreproduced adequately for CD having a protective substrate withthickness of t3. Further, the first optical path difference providingstructure on the central area separates a light converged spot of anecessary light of the third light flux which is used for recording andreproducing information, and a light converged spot of an unnecessarylight of the third light flux by the adequate distance. Thereby, thefirst optical path difference providing structure also makes thetracking characteristic good when CD is used. Additionally, the secondoptical path difference providing structure on the peripheral area cancorrect the spherochromatism (chromatic spherical aberration) for thefirst light flux and the second light flux when the wavelength isdeviated from the reference wavelength due to the reason such as themanufacturing error of the laser diode.

Examples 1-3

Next, examples which can be used for the above described embodiment willbe described. In the following examples 1-3, the objective opticalelement is a single glass lens. The objective optical element comprisesan optical surface including central area CN, the peripheral area MD,and the most peripheral area OT. The first optical path differenceproviding structure is formed on the entire surface of central area CNon the optical surface. The second optical path difference providingstructure is formed on the entire surface of the peripheral area MD onthe optical surface. The most peripheral area OT on the optical surfaceis the refractive surface in the aspheric shape.

Further, in Examples 1-3, the first optical path difference providingstructure is the structure formed by overlapping the first basicstructure and the second basic structure and having a shape in which aserrated diffractive structure and a binary structure are overlapped.The sectional shape is likely a shape shown in FIG. 2( c). The firstbasic structure which is the serrated diffractive structure is designedso as to make the light amount of the second order diffracted light fluxof the first light flux larger than the light amount of any diffractedlight flux with other diffraction order (which also includes 0-th orderflux, that is, a transmitted light flux), and to make the light amountof the first order diffracted light of the second light flux is madelarger than the light amount of any diffractive light with otherdiffraction order (which also includes 0-th order light flux, that is, atransmitted light), and to make the light amount of the first orderdiffractive light of the third light flux larger than the light amountof any diffracted light with other diffraction order (which alsoincludes 0-th order light flux, that is, a transmitted light). Further,the second basic structure which is the binary structure, is so-called,the wavelength selection diffractive structure. The second basicstructure is designed so as to make the light amount of the 0-th orderdiffracted light (transmitted light) of the first light flux larger thanthe light amount of the any diffracted light with other diffractionorder, to make the light amount of the 0-th order diffracted light(transmitted light) of the second light flux larger than the lightamount of any diffracted light with other diffraction order, and to makethe light amount of ± first order diffracted light of the third lightflux larger than the light amount of any diffracted with otherdiffraction order (which also includes 0-th order light flux, that is, atransmitted light).

Further, each of the examples 1-3 provide a first optical pathdifference providing structure including the following areas: as shownin FIG. 2( c), the area of the optical axis side of the central area inwhich the serrated structure having steps facing the optical axis sideand the binary structure are overlapped; the area of the peripheral areaside of the central area in which the serrated structure having stepsfacing the opposite side to the optical axis, and the binary structureare overlapped; the transition area arranged between the two areas,which is necessary for switching direction of the step of the serratedstructure. This transition area corresponds to a point where the opticalpath difference function has an extreme value when the optical pathdifference which is added to the transmitting wavefront by thediffractive structure, is expressed by the optical path differencefunction. Hereupon, when the optical path difference function has apoint which is the extreme value, it reduces the inclination of theoptical path difference function. Therefore, the ring-shaped zone pitchcan be extended, and the transmittance lowering due to the shape errorof the diffractive structure can be suppressed.

In the examples 1-3, the second optical path difference providingstructure is the structure formed by overlapping the first basicstructure and the fourth basic structure, and having a shape in which afiner serrated diffractive structure and a rougher serrated diffractivestructure are overlapped. Hereupon, the step faces the opposite side tothe optical axis in the finer serrated diffractive structure, and thestep faces the optical axis side in the rougher serrated diffractivestructure. The sectional shape is likely a shape shown in FIG. 2( d).The first basic structure which is the finer serrated diffractivestructure is designed so as to makes the light amount of the secondorder diffracted light of the first light flux larger than the lightamount of any diffracted light with other diffraction order (which alsoincludes 0-th order flux, that is, a transmitted light flux), to makethe light amount of the first order diffractive light of the secondlight flux larger than the light amount of any diffracted light withother diffraction order (0 which also includes 0-th order flux, that is,a transmitted light flux), and to make the light amount of the firstorder diffracted light of the third light flux larger than the lightamount of any diffracted light with other diffraction order (which alsoincludes 0-th order flux, that is, a transmitted light flux). Further,the fourth basic structure which is the rougher serrated diffractivestructure is designed so as to make the light amount of the fifth orderdiffractive light of the first light flux larger than the light amountof any diffracted light with other diffraction order, to make the lightamount of the third order diffracted light of the second light fluxlarger than the light amount of any diffracted light of otherdiffraction order, and to make the light amount of the third and secondorder diffracted light of the third light flux larger than the lightamount of any diffracted light with other diffraction order. Hereupon,in FIG. 2( c) and FIG. 2( d), the sectional shape is exaggeratinglydrawn for easy understanding.

The lens data of the examples are shown In Tables 1-1 to 6-4.Hereinafter, the power of 10 will be expressed as by using “E”. Forexample, 2.5×10⁻³ will be expressed as 2.5E-3.

Each of optical surfaces of the objective optical system is formed as anaspherical surface, which has a symmetric shape around the optical axiswith defined by substituting the coefficients shown in the tablesdescribed later into the expression 21.

$\begin{matrix}{{X(h)} = {\frac{\left( {h^{2}/r} \right)}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/r} \right)^{2}}}} + {\sum\limits_{i = 0}^{10}{A_{2i}h^{2i}}}}} & (21)\end{matrix}$

Herein, X(h) is the axis along the optical axis (the direction oftraveling light is positive direction), κ is a conical constant, A_(2i)is an aspherical coefficient, h is the height from the optical axis.

Further, The diffractive structure provides the optical path differencewith the light fluxes of respective wavelengths, which is defined bysubstituting the coefficients shown in the tables shown later into theexpression 22.

$\begin{matrix}{{\Phi (h)} = {\frac{\lambda}{\lambda_{B}} \times {dor} \times {\sum\limits_{i = 0}^{6}{C_{2i}h^{2i}}}}} & (22)\end{matrix}$

Hereupon, λ is the wavelength of the incident light flux, λ_(B) is theblaze wavelength, dor is the diffraction order, C_(2i) is thecoefficient of the optical path difference function.

Example 1

The lens data of Example 1 will be shown in Tables 1-1 to 1-3. Further,FIGS. 5( a), 5(b), and 5(c) show the vertical spherical aberrationdiagrams of Example 1. The numeral 1.0 of the vertical axis of thevertical spherical aberration diagrams expresses NA 0.85, or a diameterof 3.74 mm in BD; expresses a value slightly larger than NA 0.60, or adiameter of 2.68 mm in DVD; and expresses a value slightly larger thanNA 0.45, or a value slightly larger than a diameter of 2.18 mm in CD.Hereupon, L is 0.28 mm in Example 1. Accordingly, it providesL/F=0.28/2.42=0.116.

TABLE 1-1 Lens Data Single diffractive lens Focal length of theobjective lens f₁ = 2.20 mm f₂ = 2.28 mm f₃ = 2.42 mm Numerical apertureNA1: 0.85 NA2: 0.60 NA3: 0.45 Magnification m1: 0 m2: 0 m3: 0 The i-thsurface ri di (405 nm) ni (405 nm) di (658 nm) ni (658 nm) di (785 nm)ni (785 nm) 0 ∞ ∞ ∞ 1 (Stop 0.0 0.0 0.0 diameter) (φ3.74 mm) (φ2.68 mm)(φ2.18 mm) 2 1.5795 2.430 1.605 2.430 1.586 2.430 1.582 2-1 1.5799 2-21.5804 2-3 1.5810 2-4 1.5793 2-5 1.5816 2-6 1.5374 3 −4.7732 0.79 0.540.32 4 ∞ 0.0875 1.620 0.600 1.577 1.200 1.571 5 ∞

TABLE 1-2 Surface no. 2 2-1 2-2 2-3 Area h ≦ 1.117 1.117 ≦ h ≦ 1.1781.178 ≦ h ≦ 1.224 1.224 ≦ h ≦ 1.262 Aspheric κ −5.1483E−01 −5.1735E−01−5.1984E−01 −5.2196E−01 surface A0 0.0000E+00 −3.4971E−03 −6.9971E−03−1.0502E−02 coefficient A4 1.2950E−02 1.2950E−02 1.2950E−02 1.2952E−02A6 2.6726E−03 2.6726E−03 2.6726E−03 2.6726E−03 A8 3.8605E−03 3.8605E−033.8605E−03 3.8605E−03 A10 −3.5626E−03 −3.5626E−03 −3.5626E−03−3.5626E−03 A12 1.3965E−03 1.3965E−03 1.3965E−03 1.3965E−03 A148.5125E−05 8.5125E−05 8.5125E−05 8.5125E−05 A16 −1.6888E−04 −1.6888E−04−1.6888E−04 −1.6888E−04 A18 4.5316E−05 4.5316E−05 4.5316E−05 4.5316E−05A20 −4.4456E−06 −4.4456E−06 −4.4456E−06 −4.4456E−06 Optical pathDiffraction 2/1/1 2/1/1 2/1/1 2/1/1 difference order function *1 395 nm395 nm 395 nm 395 nm C2 −5.4912E−03 −5.4912E−03 −5.4912E−03 −5.4912E−03C4 2.3942E−03 2.3942E−03 2.3942E−03 2.3942E−03 C6 1.7081E−03 1.7081E−031.7081E−03 1.7081E−03 C8 −8.9650E−04 −8.9650E−04 −8.9650E−04 −8.9650E−04C10 2.6700E−04 2.6700E−04 2.6700E−04 2.6700E−04 Optical path Diffraction0/0/1 difference order function *1 785 nm C2 1.5968E−02 C4 −7.3725E−04C6 −1.0009E−03 C8 8.1423E−04 C10 −2.4796E−04 *1: Design wavelength

TABLE 1-3 Surface no. 2-4 2-5 2-6 3 Area 1.262 ≦ h ≦ 1.307 1.307 ≦ h ≦1.355 1.355 ≦ h Aspheric κ −5.2821E−01 −5.2092E−01 −6.5466E−01−1.4057E+01 surface A0 −1.4453E−02 −1.7566E−02 3.2124E−04 0.0000E+00coefficient A4 1.2948E−02 1.2639E−02 1.2109E−02 1.4339E−01 A6 2.6726E−032.6726E−03 6.8203E−04 −1.3569E−01 A8 3.8605E−03 3.8605E−03 1.8368E−038.5457E−02 A10 −3.5626E−03 −3.5626E−03 −1.3166E−03 −3.5414E−02 A121.3965E−03 1.3965E−03 3.0609E−04 8.3196E−03 A14 8.5125E−05 8.5125E−052.2270E−04 −8.3492E−04 A16 −1.6888E−04 −1.6888E−04 −1.6894E−040.0000E+00 A18 4.5316E−05 4.5316E−05 4.4386E−05 0.0000E+00 A20−4.4456E−06 −4.4456E−06 −4.3121E−06 0.0000E+00 Optical path Diffraction2/1/1 2/1/1 difference order function *1 395 nm 395 nm C2 −5.4912E−03−5.4912E−03 C4 2.3942E−03 2.3942E−03 C6 1.7081E−03 1.7081E−03 C8−8.9650E−04 −8.9650E−04 C10 2.6700E−04 2.6700E−04 Optical pathDiffraction difference order function *1 C2 C4 C6 C8 C10 *1: Designwavelength

Example 2

The lens data of Example 2 will be shown in Tables 2-1 to 2-3. FurtherFIGS. 6( a), 6(b), and 6(c) show the vertical spherical aberrationdiagrams of Example 2. The numeral 1.0 of the vertical axis of thevertical spherical aberration diagrams expresses NA 0.85, or a diameterof 3.74 mm in BD; expresses a value slightly larger than NA 0.60, or adiameter of 2.68 mm in DVD; and expresses a value slightly larger thanNA 0.45, or a value slightly larger than a diameter of 2.12 mm in CD.Hereupon, L is 0.16 mm in Example 2. Accordingly, it providesL/F=0.16/2.36=0.068.

TABLE 2-1 Lens Data Single diffractive lens Focal length of theobjective lens f₁ = 2.20 mm f₂ = 2.28 mm f₃ = 2.36 mm Numerical apertureNA1: 0.85 NA2: 0.60 NA3: 0.45 Magnification m1: 0 m2: 0 m3: 0 The i-thsurface ri di(405 nm) ni(405 nm) di(658 nm) ni(658 nm) di(785 nm) ni(785nm) 0 ∞ ∞ ∞ 1(Stop 0.0 0.0 0.0 diameter) (φ3.74 mm) (φ2.68 mm) (φ2.12mm) 2 1.5810 2.430 1.605 2.430 1.586 2.430 1.582 2-1 1.5825 2-2 1.58202-3 1.5809 2-4 1.5808 2-5 1.5811 2-6 1.5316 3 −4.7543 0.79 0.54 0.26 4 ∞0.0875 1.620 0.600 1.577 1.200 1.571 5 ∞

TABLE 2-2 Surface no. 2 2-1 2-2 2-3 Area h ≦ 1.116 1.116 ≦ h ≦ 1.1971.197 ≦ h ≦ 1.239 1.239 ≦ h ≦ 1.273 Aspheric κ −5.0687E−01 −5.0696E−01−5.1166E−01 −5.1724E−01 surface A0 0.0000E+00 −3.3441E−03 −7.0033E−03−1.0810E−02 coefficient A4 1.3630E−02 1.3630E−02 1.3630E−02 1.3630E−02A6 2.1948E−03 2.1948E−03 2.1948E−03 2.1948E−03 A8 3.9803E−03 3.9803E−033.9803E−03 3.9803E−03 A10 −3.7498E−03 −3.7498E−03 −3.7498E−03−3.7498E−03 A12 1.4932E−03 1.4932E−03 1.4932E−03 1.4932E−03 A147.7535E−05 7.7535E−05 7.7535E−05 7.7535E−05 A16 −1.6888E−04 −1.6888E−04−1.6888E−04 −1.6888E−04 A18 4.5316E−05 4.5316E−05 4.5316E−05 4.5316E−05A20 −4.4456E−06 −4.4456E−06 −4.4456E−06 −4.4456E−06 Optical pathDiffraction 2/1/1 2/1/1 2/1/1 2/1/1 difference order function *1 395 nm395 nm 395 nm 395 nm C2 −5.5507E−03 −5.5507E−03 −5.5507E−03 −5.5507E−03C4 2.6083E−03 2.6083E−03 2.6083E−03 2.6083E−03 C6 1.6957E−03 1.6957E−031.6957E−03 1.6957E−03 C8 −1.0148E−03 −1.0148E−03 −1.0148E−03 −1.0148E−03C10 3.1382E−04 3.1382E−04 3.1382E−04 3.1382E−04 Optical path Diffraction0/0/1 difference order function *1 785 nm C2 1.0109E−02 C4 −5.6774E−04C6 −1.8990E−03 C8 1.5121E−03 C10 −4.9017E−04 *1: Design wavelength

TABLE 2-3 Surface no. 2-4 2-5 2-6 3 Area 1.273 ≦ h ≦ 1.316 1.316 ≦ h ≦1.351 1.351 ≦ h Aspheric κ −5.2032E−01 −5.2211E−01 −6.5466E−01−1.4567E+01 surface A0 −1.4468E−02 −1.8188E−02 2.0102E−03 0.0000E+00coefficient A4 1.3630E−02 1.3629E−02 1.2109E−02 1.4319E−01 A6 2.1948E−032.1948E−03 6.8203E−04 −1.3589E−01 A8 3.9803E−03 3.9803E−03 1.8368E−038.5430E−02 A10 −3.7498E−03 −3.7498E−03 −1.3166E−03 −3.5356E−02 A121.4932E−03 1.4932E−03 3.0609E−04 8.2999E−03 A14 7.7536E−05 7.7535E−052.2270E−04 −8.3293E−04 A16 −1.6888E−04 −1.6888E−04 −1.6894E−040.0000E+00 A18 4.5316E−05 4.5316E−05 4.4386E−05 0.0000E+00 A20−4.4456E−06 −4.4456E−06 −4.3121E−06 0.0000E+00 Optical path Diffraction2/1/1 2/1/1 difference order function *1 395 nm 395 nm C2 −5.5507E−03−5.5507E−03 C4 2.6083E−03 2.6083E−03 C6 1.6957E−03 1.6957E−03 C8−1.0148E−03 −1.0148E−03 C10 3.1382E−04 3.1382E−04 Optical pathDiffraction difference order function *1 C2 C4 C6 C8 C10 *1: Designwavelength

Example 3

The lens data of Example 31 will be shown in Tables 3-1 to 3-3. Further,FIGS. 7( a), 7(b), and 7(c) show the vertical spherical aberrationdiagrams of Example 2. The numeral 1.0 of the vertical axis of thevertical spherical aberration diagram expresses NA 0.85 or a diameter of3.74 mm in BD, expresses a value slightly larger than NA 0.60, or adiameter of 2.68 mm in DVD, and expresses a value slightly larger thanNA 0.45, or a value slightly larger than a diameter of 2.17 mm in CD.Hereupon, L is 0.28 mm in Example 3. Accordingly, it providesL/f=0.28/2.43=0.115.

TABLE 3-1 Lens Data Single diffraction lens Focal length of theobjective lens f₁ = 2.20 mm f₂ = 2.28 mm f₃ = 2.43 mm Numerical apertureNA1: 0.85 NA2: 0.60 NA3: 0.45 Magnification m1: 0 m2: 0 m3: 0 The i-thsurface ri di(405 nm) ni(405 nm) di(658 nm) ni(658 nm) di(785 nm) ni(785nm) 0 ∞ ∞ ∞ 1(Stop 0.0 0.0 0.0 diameter) (φ3.74 mm) (φ2.68 mm) (φ2.17mm) 2 1.5559 2.370 1.605 2.370 1.586 2.370 1.582 2-1 1.5627 2-2 1.56532-3 1.5643 2-4 1.5658 2-5 1.5648 3 −5.1533 0.82 0.57 0.36 4 ∞ 0.08751.620 0.600 1.577 1.200 1.571 5 ∞

TABLE 3-2 Surface no. 2 2-1 2-2 2-3 Area h ≦ 1.110 1.110 ≦ h ≦ 1.1791.179 ≦ h ≦ 1.218 1.218 ≦ h ≦ 1.278 Aspheric κ −5.3140E−01 −5.2257E−01−5.2131E−01 −5.2861E−01 surface A0 0.0000E+00 −6.0616E−03 −9.0884E−03−1.2687E−02 coefficient A4 1.2538E−02 1.2538E−02 1.2538E−02 1.2538E−02A6 1.5730E−03 1.5730E−03 1.5730E−03 1.5730E−03 A8 7.8213E−03 7.8213E−037.8213E−03 7.8213E−03 A10 −6.6801E−03 −6.6801E−03 −6.6801E−03−6.6801E−03 A12 2.5319E−03 2.5319E−03 2.5319E−03 2.5319E−03 A14−7.3283E−05 −7.3283E−05 −7.3283E−05 −7.3283E−05 A16 −1.6888E−04−1.6888E−04 −1.6888E−04 −1.6888E−04 A18 4.5316E−05 4.5316E−05 4.5316E−054.5316E−05 A20 −4.4456E−06 −4.4456E−06 −4.4456E−06 −4.4456E−06 Opticalpath Diffraction 2/1/1 2/1/1 2/1/1 2/1/1 difference order function *1395 nm 395 nm 395 nm 395 nm C2 −4.2889E−03 −4.2889E−03 −4.2889E−03−4.2889E−03 C4 1.6784E−03 1.6784E−03 1.6784E−03 1.6784E−03 C6 2.4623E−032.4623E−03 2.4623E−03 2.4623E−03 C8 −1.1484E−03 −1.1484E−03 −1.1484E−03−1.1484E−03 C10 3.0281E−04 3.0281E−04 3.0281E−04 3.0281E−04 Optical pathDiffraction 0/0/1 difference order function *1 785 nm C2 1.6209E−02 C4−4.2084E−04 C6 −1.8779E−03 C8 1.5357E−03 C10 −4.6042E−04 *1: Designwavelength

TABLE 3-3 Surface no. 2-4 2-5 3 Area 1.278 ≦ h ≦ 1.346 1.346 ≦ hAspheric surface κ −5.3306E−01 −6.5475E−01 −9.8372E+00 coefficient A0−1.9455E−02 8.6537E−02 0.0000E+00 A4 1.2538E−02 1.5782E−02 1.4197E−01 A61.5730E−03 9.9623E−04 −1.3547E−01 A8 7.8213E−03 1.7248E−03 8.6460E−02A10 −6.6801E−03 −1.3501E−03 −3.5392E−02 A12 2.5319E−03 3.0344E−048.0551E−03 A14 −7.3283E−05 2.2480E−04 −7.7385E−04 A16 −1.6888E−04−1.6807E−04 0.0000E+00 A18 4.5316E−05 4.4386E−05 0.0000E+00 A20−4.4456E−06 −4.3584E−06 0.0000E+00 Diffraction order 2/1/1 Optical path*1 395 nm difference function C2 −4.2889E−03 C4 1.6784E−03 C6 2.4623E−03C8 −1.1484E−03 C10 3.0281E−04 Optical path *1 difference function C2 C4C6 C8 C10 *1: Design wavelength

Example 4

In Example 4, the objective optical element is a single plastic lensformed of polyolefin. The objective optical element comprises an opticalsurface comprising the central area CN, the peripheral area MD, and themost peripheral area OT. The first optical path difference providingstructure is formed on the entire surface of central area CN on theoptical surface. The second optical path difference providing structureis formed on the entire surface of the peripheral area MD on the opticalsurface. The third optical path difference providing structure is formedon the entire surface of the most peripheral area OT on the opticalsurface.

Further, in the Example 4, the first optical path difference providingstructure is the structure formed by overlapping the first basicstructure, the second basic structure and the third basic structure, andhaving a shape in which two kinds of the serrated diffractive structuresand a binary structure are overlapped. The sectional shape of the firstoptical path difference providing structure is shown as a portionrepresented as CN in FIG. 8. The third basic structure is a serrateddiffractive structure and is designed so as to make the light amount ofthe tenth order diffracted light of the first light flux larger than thelight amount of any diffractive light with other diffraction order(which also includes 0-th order flux, that is, a transmitted lightflux), to make the light amount of the sixth order diffracted light ofthe second light flux larger than the light amount of any diffractedlight with other diffraction order (which also includes 0-th order flux,that is, a transmitted light flux), and to make the light amount of thefifth order diffracted light of the third light flux larger than thelight amount of any diffracted light with other diffraction order (whichalso includes 0-th order flux, that is, a transmitted light flux). Thestep amount in the direction of the optical axis of the step in thefirst basic structure is the step amount providing optical pathdifference which is about two times of the first wavelength for thefirst light flux, also providing optical path difference which is about1.2 times of the second wavelength for the second light flux and furtherproviding optical path difference which is about 1 time of the thirdwavelength for the third light flux. The step amount in the direction ofthe optical axis of the step in the second basic structure is the stepamount providing optical path difference which is about five times ofthe first wavelength for the first light flux, also providing opticalpath difference which is about three times of the second wavelength forthe second light flux and further providing optical path differencewhich is about 2.5 times of the third wavelength for the third lightflux. The step amount in the direction of the optical axis of the stepin the third basic structure is the step amount providing optical pathdifference which is about ten times of the first wavelength for thefirst light flux, also providing optical path difference which is aboutsix times of the second wavelength for the second light flux and furtherproviding optical path difference which is about five times of the thirdwavelength for the third light flux. The third basic structure isprovided on a basic aspheric surface which is different from a basicaspheric surface on which the first basic structure and the second basicstructure are provided.

In Example 4, the second optical path difference providing structure isthe structure formed by overlapping the first basic structure and thefourth basic structure as shown as MD in FIG. 8, and has a shape inwhich two kinds of the serrated diffractive structures are overlapped.The step amount in the direction of the optical axis of the step in thefourth basic structure is the step amount providing optical pathdifference which is about five times of the first wavelength for thefirst light flux, also providing optical path difference which is aboutthree times of the second wavelength for the second light flux andfurther providing optical path difference which is about 2.5 times ofthe third wavelength for the third light flux. The fourth basicstructure is provided on a basic aspheric surface which is differentfrom a basic aspheric surface on which the first basic structure isprovided. The third basic structure in the first optical path differenceproviding structure and the fourth basic structure in the second opticalpath difference providing structure are continuous. The third basicstructure and the fourth basic structure are the structure in which thedepth of the third basic structure in the direction of the optical axisbecomes deeper as farther from the optical axis in the directionperpendicular to the optical axis till the boundary between the firstoptical path difference providing structure and the second optical pathdifference providing structure, and then in the area farther from theoptical axis than the boundary, the depth of the fourth basic structurein the direction of the optical axis becomes shallower as farther fromthe optical axis in the direction perpendicular to the optical axis.

In the Example 4, the third optical path difference providing structureis the structure having only the fourth basic structure as shown as OTin FIG. 8, and has a shape which has only one kind of serrateddiffractive structure. The third basic structure in the third opticalpath difference providing structure is not the structure in which thedepth of the structure in the direction of the optical axis becomesdeeper as farther from the optical axis in the direction perpendicularto the optical axis till one point, and then in the area farther fromthe optical axis than the one point, the depth of the structure in thedirection of the optical axis becomes shallower as farther from theoptical axis in the direction perpendicular to the optical axis.

The lens data of Example 4 will be shown in Tables 4-1 to 4-4. Further,FIGS. 9( a) to 9(c) shows the vertical spherical aberration diagrams ofExample 4. The numeral 1.0 of the vertical axis of the verticalspherical aberration diagrams expresses NA 0.85 or a diameter of 3.74 mmin BD, expresses a value slightly larger than NA 0.60, or a valueslightly larger than a diameter of 2.70 mm in DVD, and expresses a valueslightly larger than NA 0.45, or a value slightly larger than a diameterof 2.37 mm in CD. Hereupon, L is 0.60 mm in Example 4. Accordingly, itprovides L/f=0.60/2.53=0.237.

The entire of ring-shaped zones of the first optical path differenceproviding structure in Example 4 can be classified to the groups: agroup of a step amount in the range of 3.62 μm to 4.23 μm; and a groupof a step amount in the range of 2.22 μm to 2.56 μm. Here, λB is 405 nm.λB′ is an arbitral value in the range of 390 nm to 400 nm. Accordingly,each of the entire ring shaped zones in the first optical pathdifference providing structure in Example 4 has a step amount satisfyingany one of dC and dD. Each of the entire ring shaped zones in the firstoptical path difference providing structure has a pitch differencewithin the range of 5.3 μm to 110 μm. Each of the entire ring shapedzones in the first optical path difference providing structure has avalue of (step amount/pitch width) being 0.8 or less.

TABLE 4-1 Lens Data Single diffraction lens Focal length of theobjective lens f₁ = 2.20 mm f₂ = 2.28 mm f₃ = 2.53 mm Numerical apertureNA1: 0.85 NA2: 0.60 NA3: 0.45 Magnification m1: 0 m2: 0 m3: 0 The i-thsurface ri di(405 nm) ni(405 nm) di(658 nm) ni(658 nm) di(785 nm) ni(785nm) 0 ∞ ∞ ∞ 1(Stop 0.0 0.0 0.0 diameter) (φ3.74 mm) (φ2.70 mm) (φ2.37mm) 2 1.5656 2.680 1.561 2.680 1.540 2.680 1.536 2-1 1.5626 2-2 1.55952-3 1.5570 2-4 1.5661 2-5 1.5633 2-6 1.5643 2-7 1.5657 2-8 1.5656 2-91.53232 3 −2.8740 0.67 0.43 0.41 4 ∞ 0.0875 1.620 0.600 1.577 1.2001.571 5 ∞

TABLE 4-2 Surface no. 2 2-1 2-2 2-3 Area h ≦ 0.3982 0.3982 ≦ h ≦ 0.63920.6392 ≦ h ≦ 0.9173 0.9173 ≦ h ≦ 1.2020 Aspheric κ −0.545763E+00−0.544149E+00 −0.543545E+00 −0.540372E+00 surface A0  0.000000E+00 0.723148E−02  0.144639E−01  0.217471E−01 coefficient A4  0.173456E−01 0.173456E−01  0.173456E−01  0.173456E−01 A6  0.161268E−02  0.161268E−02 0.161268E−02  0.161268E−02 A8  0.227272E−02  0.227272E−02  0.227272E−02 0.227272E−02 A10 −0.176212E−02 −0.176212E−02 −0.176212E−02−0.176212E−02 A12  0.832672E−03  0.832672E−03  0.832672E−03 0.832672E−03 A14  0.306247E−03  0.306247E−03  0.306247E−03 0.306247E−03 A16 −0.312510E−03 −0.312510E−03 −0.312510E−03−0.312510E−03 A18  0.779196E−04  0.779196E−04  0.779196E−04 0.779196E−04 A20 −0.382183E−05 −0.382183E−05 −0.382183E−05−0.382183E−05 Optical Diffraction 2/1/1 2/1/1 2/1/1 2/1/1 path orderdifference *1 395 nm 395 nm 395 nm 395 nm function C2  −7.9481E−03 −7.9481E−03  −7.9481E−03  −7.9481E−03 C4   3.1618E−03   3.1618E−03  3.1618E−03   3.1618E−03 C6   2.6104E−04   2.6104E−04   2.6104E−04  2.6104E−04 C8  −1.5449E−04  −1.5449E−04  −1.5449E−04  −1.5449E−04 C10  1.3011E−04   1.3011−04   1.3011E−04   1.3011E−04 Optical Diffraction0/0/1 0/0/1 0/0/1 0/0/1 path order difference *1 785 nm 785 nm 785 nm785 nm function C2   3.2600E−02   3.2600E−02   3.2600E−02   3.2600E−02C4  −3.0280E−03  −3.0280E−03  −3.0280E−03  −3.0280E−03 C6   2.4526E−03  2.4526E−03   2.4526E−03   2.4526E−03 C8  −1.0989E−03  −1.0989E−03 −1.0989E−03  −1.0989E−03 C10   2.4093E−04   2.4093E−04   2.4093E−04  2.4093E−04 *1: Design wavelength

TABLE 4-3 Surface no. 2-4 2-5 2-6 2-7 Area 1.2020 ≦ h ≦ 1.2390 1.2390 ≦h ≦ 1.2677 1.2677 ≦ h ≦ 1.3121 1.3121 ≦ h ≦ 1.3466 Aspheric κ−0.523735E+00 −0.534676E+00 −0.540742E+00 −0.536630E+00 surface A0 0.193505E−01  0.150921E−01  0.113044E−01  0.782945E−02 coefficient A4 0.173456E−01  0.173485E−01  0.175724E−01  0.172773E−01 A6  0.161268E−02 0.161268E−02  0.161268E−02  0.161268E−02 A8  0.227272E−02  0.227272E−02 0.227272E−02  0.227272E−02 A10 −0.176212E−02 −0.176212E−02−0.176212E−02 −0.176212E−02 A12  0.832672E−03  0.832672E−03 0.832672E−03  0.832672E−03 A14  0.306247E−03  0.306247E−03 0.306247E−03  0.306247E−03 A16 −0.312510E−03 −0.312510E−03−0.312510E−03 −0.312510E−03 A18  0.779196E−04  0.779196E−04 0.779196E−04  0.779196E−04 A20 −0.382183E−05 −0.382183E−05−0.382183E−05 −0.382183E−05 Optical Diffraction 2/1/1 2/1/1 2/1/1 2/1/1path order difference *1 395 nm 395 nm 395 nm 395 nm function C2 −7.9481E−03  −7.9481E−03  −7.9481E−03  −7.9481E−03 C4   3.1618E−03  3.1618E−03   3.1618E−03   3.1618E−03 C6   2.6104E−04   2.6104E−04  2.6104E−04   2.6104E−04 C8  −1.5449E−04  −1.5449E−04  −1.5449E−04 −1.5449E−04 C10   1.3011E−04   1.3011E−04   1.3011E−04   1.3011E−04Optical Diffraction path order difference *1 function C2 C4 C6 C8 C10*1: Design wavelength

TABLE 4-4 Surface no. 2-8 2-9 3 Area 1.3466 ≦ h ≦ 1.3751 1.3751 ≦ hAspheric κ −0.545757E+00   −0.616167E+00   −5.4022E+01   surface A00.150988E−04 0.341800E−01 0.0000E+00 coefficient A4 0.173456E−010.132229E−01 1.0541E−01 A6 0.161240E−02 0.544502E−04 −1.0213E−01   A80.227272E−02 0.262231E−02 7.4675E−02 A10 −0.176212E−02   −0.156680E−02  −4.3240E−02   A12 0.832672E−03 0.226928E−03 1.4629E−02 A14 0.306247E−030.239248E−03 −2.0762E−03   A16 −0.312510E−03   −0.165881E−03  0.0000E+00 A18 0.779196E−04 0.451501E−04 0.0000E+00 A20 −0.382183E−05  −0.472873E−05   0.0000E+00 Diffraction order 2/1/1 5/3/2 Optical path *1395 nm 405 nm difference C2  −7.9481E−03  −1.0012E−03 function C4 3.1618E−03  −1.0849E−04 C6  2.6104E−04  1.2384E−05 C8  −1.5449E−04 −5.9681E−06 C10  1.3011E−04  −8.9463E−06 Optical path *1 difference C2function C4 C6 C8 C10 *1: Design wavelength

Further, as for the temperature characteristics of the objective opticalelement in the Example 4, δSAT1 is +0.0033 WFEλ rms/° C., and δSAT2 is+0.0019 WFEλ rms/° C. Further, because f of the objective opticalelement for the first wavelength is 2.2 mm, δSAT1/f is +0.0015 WFEλrms/(° C.·mm), and δSAT2/f is +0.0009 WFEλ rms/(° C.·mm). Further, asfor the wavelength characteristics of the objective optical element ofthe Example 4, δSAλ is −0.03 λrms/nm, and δSAλ/f is −0.0136λrms/(nm·mm). Hereupon, the using wavelength is 405 nm, and the ambienttemperature for the wavelength characteristics is 25° C.

Furthermore, in case that there is provided collimator lens CL which isa single lens formed of the same material (polyolefin plastic) as theobjective optical element, and the collimator lens is used for Example 4with being combined with the objective optical element in Example 4,δSAT3 is +0.0004 WFEλ rms/° C., and δSAT3/f is +0.0002 WFEλ rms/(°C.·mm). The lens data of the collimator lens will be shown in Table 5.

TABLE 5 Lens Data Plastic Normal Collimator Focal length of thecollimating lens f₁ = 17.5 mm, f₂ = 18.1 mm, f₃ = 18.2 mm The i-thsurface ri di(405 nm) ni(405 nm) di(658 nm) ni(658 nm) di(785 nm) ni(785nm) 0 ∞ ∞ ∞ ∞ 1(Stop ∞ 0.0 0.0 0.0 diameter) (φ4.2 mm) (φ4.2 mm) (φ4.2mm) 2  10.9657 1.900 1.545 1.900 1.527 1.900 1.524 3 −68.6939 5.0005.000 5.000 4 ∞ 8.000 1.530 8.000 1.514 8.000 1.511 5 ∞ 6.145 6.6616.764 6 ∞ Surface no. 2 3 Aspheric surface κ −6.3761E−01 −3.8132E+01coefficient

Example 5

In Example 5, the objective optical element is a single plastic lensformed of polyolefin. The objective optical element comprises an opticalsurface comprising central area CN, the peripheral area MD, and the mostperipheral area OT. The first optical path difference providingstructure is formed on the entire surface of central area CN on theoptical surface. The second optical path difference providing structureis formed on the entire surface of the peripheral area MD on the opticalsurface. The third optical path difference providing structure is formedon the most peripheral area OT on the entire surface of the opticalsurface. The sectional shape is the shape likely to FIG. 8.

Further, in Example 5, the first optical path difference providingstructure is the structure formed by overlapping the third basicstructure with the first basic structure and the second basic structure,and having a shape in which two kinds of the serrated diffractivestructures and the binary structure are overlapped.

In Example 5, the second optical path difference providing structure isthe structure formed by overlapping the first basic structure and thefourth basic structure, and having a shape in which two kinds ofserrated diffractive structures are overlapped.

In Example 5, the third optical path difference providing structure isthe structure formed by only the fourth basic structure, and having ashape including one kind of the serrated diffractive structure.

The lens data of Example 5 will be shown in Tables 6-1 to 6-4. Further,FIG. 11 shows vertical spherical aberration diagrams of Example. Thenumeral 1.0 of the vertical axis of the vertical spherical aberrationdiagrams expresses NA 0.85, or a diameter of 3.74 mm in BD, expresses avalue slightly larger than 0.60, or a value slightly larger than adiameter of 2.71 mm in DVD, and expresses a value slightly larger thanNA 0.45, or a value slightly larger than a diameter of 2.24 mm in CD.Hereupon, L is 0.38 mm in Example 5. Accordingly, it providesL/f=0.38/2.45=0.155.

TABLE 6-1 Lens Data Single diffraction lens Focal length of theobjective lens f₁ = 2.20 mm f₂ = 2.29 mm f₃ = 2.45 mm Numerical apertureNA1: 0.85 NA2: 0.60 NA3: 0.45 Magnification m1: 0 m2: 0 m3: 0 The i-thsurface ri di(405 nm) ni(405 nm) di(658 nm) ni(658 nm) di(785 nm) ni(785nm) 0 ∞ ∞ ∞ ∞ 1(Stop ∞ 0.0 0.0 0.0 diameter) (φ3.74 mm) (φ2.71 mm)(φ2.24 mm) 2 1.5827 2.530 1.560 2.530 1.541 2.530 1.537 2-1 1.5799 2-21.5769 2-3 1.5741 2-4 1.5816 2-5 1.5816 2-6 1.5827 2-7 1.5824 2-8 1.51232-9 1.5126 2-10 1.5126 3 −3.3232  0.74 0.51 0.34 4 ∞ 0.0875 1.620 0.6001.577 1.200 1.571 5 ∞

TABLE 6-2 Surface no. 2 2-1 2-2 2-3 Area h ≦ 0.357 0.357 ≦ h ≦ 0.5630.563 ≦ h ≦ 0.779 0.779 ≦ h ≦ 1.229 Aspheric κ −5.1901E−01 −5.1920E−01−5.1610E−01 −5.1369E−01 surface A0 0.0000E+00 7.2386E−03 1.4474E−022.1741E−02 coefficient A4 1.6812E−02 1.6812E−02 1.6812E−02 1.6812E−02 A67.2184E−04 7.2184E−04 7.2184E−04 7.2184E−04 A8 3.6921E−03 3.6921E−033.6921E−03 3.6921E−03 A10 −2.3832E−03 −2.3832E−03 −2.3832E−03−2.3832E−03 A12 7.3118E−04 7.3118E−04 7.3118E−04 7.3118E−04 A144.3460E−04 4.3460E−04 4.3460E−04 4.3460E−04 A16 −3.3859E−04 −3.3859E−04−3.3859E−04 −3.3859E−04 A18 7.7905E−05 7.7905E−05 7.7905E−05 7.7905E−05A20 −3.8236E−06 −3.8236E−06 −3.8236E−06 −3.8236E−06 Optical pathDiffraction 2/1/1 2/1/1 2/1/1 2/1/1 difference order function *1 395 nm395 nm 395 nm 395 nm C2 −9.6690E−03 −9.6690E−03 −9.6690E−03 −9.6690E−03C4 2.8071E−03 2.8071E−03 2.8071E−03 2.8071E−03 C6 4.5147E−04 4.5147E−044.5147E−04 4.5147E−04 C8 −1.8776E−04 −1.8776E−04 −1.8776E−04 −1.8776E−04C10 1.1487E−04 1.1487E−04 1.1487E−04 1.1487E−04 Optical path Diffraction0/0/1 0/0/1 0/0/1 0/0/1 difference order function *1 785 nm 785 nm 785nm 785 nm C2 2.1432E−02 2.1432E−02 2.1432E−02 2.1432E−02 C4 −1.7579E−03−1.7579E−03 −1.7579E−03 −1.7579E−03 C6 6.1798E−04 6.1798E−04 6.1798E−046.1798E−04 C8 −1.7907E−04 −1.7907E−04 −1.7907E−04 −1.7907E−04 C105.5149E−06 5.5149E−06 5.5149E−06 5.5149E−06 *1: Design wavelength

TABLE 6-3 Surface no. 2-4 2-5 2-6 2-7 Area 1.229 ≦ h ≦ 1.279 1.279 ≦ h ≦1.313 1.313 ≦ h ≦ 1.338 1.338 ≦ h ≦ 1.360 Aspheric κ −5.0865E−01−5.1138E−01 −5.0846E−01 −5.1704E−01 surface A0 1.9053E−02 1.5184E−021.1708E−02 7.7827E−03 coefficient A4 1.7171E−02 1.7096E−02 1.6812E−021.6882E−02 A6 7.2184E−04 7.2184E−04 7.2184E−04 7.8583E−04 A8 3.6921E−033.6921E−03 3.6921E−03 3.6921E−03 A10 −2.3832E−03 −2.3832E−03 −2.3832E−03−2.3832E−03 A12 7.3118E−04 7.3118E−04 7.3118E−04 7.3118E−04 A144.3460E−04 4.3460E−04 4.3460E−04 4.3460E−04 A16 −3.3859E−04 −3.3859E−04−3.3859E−04 −3.3859E−04 A18 7.7905E−05 7.7905E−05 7.7905E−05 7.7905E−05A20 −3.8236E−06 −3.8236E−06 −3.8236E−06 −3.8236E−06 Optical pathDiffraction 2/1/1 2/1/1 2/1/1 2/1/1 difference order function *1 395 nm395 nm 395 nm 395 nm C2 −9.6690E−03 −9.6690E−03 −9.6690E−03 −9.6690E−03C4 2.8071E−03 2.8071E−03 2.8071E−03 2.8071E−03 C6 4.5147E−04 4.5147E−044.5147E−04 4.5147E−04 C8 −1.8776E−04 −1.8776E−04 −1.8776E−04 −1.8776E−04C10 1.1487E−04 1.1487E−04 1.1487E−04 1.1487E−04 Optical path Diffractiondifference order function *1 C2 C4 C6 C8 C10 *1: Design wavelength

TABLE 6-4 Surface no. 2-8 2-9 2-10 3 Area 1.360 ≦ h ≦ 1.693 1.693 ≦ h ≦1.795 h ≧ 1.795 Aspheric κ −6.3971E−01 −6.3971E−01 −6.3971E−01−6.4455E+01 surface A0 3.3772E−02 3.3772E−02 3.3772E−02 0.0000E+00coefficient A4 6.7776E−03 6.7776E−03 6.7776E−03 1.0127E−01 A6 9.8447E−049.8447E−04 9.8447E−04 1.0483E−01 A8 3.0474E−03 3.0474E−03 3.0474E−038.1714E−02 A10 −1.6077E−03 −1.6077E−03 −1.6077E−03 −4.5164E−02 A121.9240E−04 1.9240E−04 1.9240E−04 1.3855E−02 A14 2.3762E−04 2.3762E−042.3762E−04 1.7611E−03 A16 −1.6317E−04 −1.6317E−04 −1.6317E−04 0.0000E+00A18 4.5602E−05 4.5602E−05 4.5602E−05 0.0000E+00 A20 −4.8919E−06−4.8919E−06 −4.8919E−06 0.0000E+00 Optical path Diffraction 5/3/2 2/1/15/3/2 difference order function *1 405 nm 405 nm 405 nm C2 −2.9493E−03−7.3734E−03 −2.9493E−03 C4 2.1334E−04 5.3335E−04 2.1334E−04 C6−3.0344E−05 −7.5859E−05 −3.0344E−05 C8 −2.1699E−05 −5.4247E−05−2.1699E−05 C10 −4.3323E−06 −1.0831E−05 −4.3323E−06 Optical pathDiffraction difference order function *1 C2 C4 C6 C8 C10 *1: Designwavelength

Further, as for the temperature characteristics of the objective opticalelement of Example 5, δSAT1 is +0.0308 WFEλ rms/° C., and δSAT2 is+0.00176 WFEλ rms/° C. Further, because f of the objective opticalelement for the first wavelength is 2.20 mm, δSAT1/f is +0.0014 WFEλrms/(° C. ·mm), and δSAT2/f is +0.0008 WFEλ rms/(° C.·mm). Further, asfor the wavelength characteristics of the objective optical element ofExample 5, δSAλ is −0.02618 λrms/nm, and δSAλ/f is −0.0119 λrms/(nm·mm).Hereupon, the using wavelength is 405 nm, and the ambient temperaturefor the wavelength characteristics is 25° C.

Furthermore, in case that there is provided collimator lens CL which isa single lens same as the collimator lens used in Example 4, and thecollimator lens is used for Example 5 with being combined with theobjective optical element in Example 5, δSAT3 is +0.000198 WFEλ rms/°C., and δSAT3/f is +0.00009 WFEλ rms/(° C.·mm).

Next, an example of the manufacturing method of the optical pickupapparatus by using the objective optical element of Example 4 or 5, andthe similar objective optical element to them, will be described.Herein, it is considered that the objective optical element is optimizedto the blue-violet semiconductor laser LD1 in which the deviation of theoscillation wavelength is larger. Initially, it is defined that thereference wavelength of the blue-violet semiconductor laser LD1 is 405nm. The objective optical element in the first group is formed asfollows. The objective optical element is designed, for the referencewavelength, so as to include an optical path difference providingstructure by which the spherical aberration becomes optimum when thedesign wavelength is 402 nm. The first metallic mold corresponding tothe objective optical element is produced, and the objective opticalelement molded by the first metallic mold is labeled as the first group.

The objective optical element in the second group is formed as follows.The objective optical element is designed so as to include an opticalpath difference providing structure by which the spherical aberrationbecomes optimum when the design wavelength is 403 nm. The secondmetallic mold corresponding to the objective optical element isproduced, and the objective optical element molded by the secondmetallic mold is labeled as the second group.

The objective optical element in the third group is formed as follows.The objective optical element is designed so as to include an opticalpath difference providing structure by which the spherical aberrationbecomes optimum when the design wavelength is 404 nm. The third metallicmold corresponding to the objective optical element is produced, and theobjective optical element molded by the third metallic mold is labeledas the third group.

The objective optical element in the fourth group is formed as follows.The objective optical element is designed so as to include an opticalpath difference providing structure by which the spherical aberrationbecomes optimum when the design wavelength is 405 nm. The fourthmetallic mold corresponding to the objective optical element isproduced, and the objective optical element molded by the fourthmetallic mold is labeled as the fourth group.

The objective optical element in the fifth group is formed as follows.The objective optical element is designed so as to include an opticalpath difference providing structure by which the spherical aberrationbecomes optimum when the design wavelength is 406 nm. The fifth metallicmold corresponding to the objective optical element is produced, and theobjective optical element molded by the fifth metallic mold is labeledas the fifth group.

The objective optical element in the sixth group is formed as follows.The objective optical element is designed so as to include an opticalpath difference providing structure by which the spherical aberrationbecomes optimum when the design wavelength is 407 nm. The sixth metallicmold corresponding to the objective optical element is produced, and theobjective optical element molded by the sixth metallic mold is labeledas the sixth group.

The objective optical element in the seventh group is formed as follows.The objective optical element is designed so as to include an opticalpath difference providing structure by which the spherical aberrationbecomes optimum when the design wavelength is 408 nm. The seventhmetallic mold corresponding to the objective optical element isproduced, and the objective optical element molded by the seventhmetallic mold is labeled as the seventh group.

In this embodiment, the specification of the optical path differenceproviding structure is divided into seven groups. However it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. For example, the numbers of groups may be 3 or5.

FIG. 15 is a perspective view of the molded objective lens OBJ. As shownin FIG. 15, a convex or concave identification mark M is formed on thecircular flange F arranged around the optical surface OP in theobjective optical element OBJ. The mark can be simultaneouslytransferred and formed on the objective optical element OBJ at the timeof the molding of the objective optical element OBJ by previouslyforming concave portion or convex portion corresponding to the mark onthe flange transfer surface of the metallic mold (not shown). In thepresent embodiment, when one identification mark M is formed, it showsthat the objective lens belongs to the first group. When twoidentification marks M are formed, it shows that the objective lensbelongs to the second group. When three identification marks M areformed, it shows that the objective lens belongs to the third group. Thesucceeding groups are identified by the same manner. Hereupon, a methodof labeling the groups is not limited to the above method. For example,the different identification mark may also be attached on that tray ofan objective optical lens, cartridge or box for packaging that.

FIG. 16 is a flowchart showing the manufacturing method of the opticalpickup apparatus according to the present embodiment. In step S101 ofFIG. 16, the oscillation wavelength λ1 of the arbitrary blue-violetsemiconductor laser diode is measured. Next, in step S102, if themeasured oscillation wavelength λ1 is 401.5 nm or more, and less than402.5 nm, n is set to 1; if the measured oscillation wavelength λ1 is402.5 nm or more, and less than 403.5 nm, n is set to 2; if the measuredoscillation wavelength λ1 is 403.5 nm or more, and less than 404.5 nm, nis set to 3; when the measured oscillation wavelength λ1 is 404.5 nm ormore, and less than 405.5 nm, n is set to 4; if the measured oscillationwavelength λ1 is 405.5 nm or more, and less than 406.5 nm, n is set to5; when the measured oscillation wavelength λ1 is 406.5 nm or more, andless than 407.5 nm, n is set to 6; and if the measured oscillationwavelength λ1 is 407.5 nm or more, and less than 408.5 nm, n is set to7. Hereupon, when the oscillation wavelength λ1 of the blue-violetsemiconductor laser is less than 401.5 nm, or is 408.5 nm or more, itmay be replaced with another one as the product which is out of thetolerance.

In succeeding step S103, the objective optical element in the n-th groupis selected. Further, in step S104, when the measured blue-violetsemiconductor laser diode and parts including the selected objectiveoptical element are assembled, the optical pickup apparatus iscompleted.

The present invention is not limited to the examples described in thespecification, it is apparent to those skilled in the art, from theexamples and spirit described in the present specification that theinvention includes other examples and modified examples. The descriptionand examples in the specification is persistently purposed for theexemplification, and the scope of the present invention is shown by thefollowing claims.

1. An optical pickup apparatus for recording and/or reproducinginformation for an optical disk, the optical pickup apparatuscomprising: a first light source for emitting a first light flux havinga first wavelength λ1; a second light source for emitting a second lightflux having a second wavelength λ2 (λ2>λ1); a third light source foremitting a third light flux having a third wavelength λ3 (λ3>λ2); and anobjective optical element for converging the first light flux onto aninformation recording surface of a first optical disk having aprotective substrate with a thickness t1, for converging the secondlight flux onto an information recording surface of a second opticaldisk having a protective substrate with a thickness t2 (t1≦t2), and forconverging the third light flux onto an information recording surface ofa third optical disk having a protective substrate with a thickness t3(t2<t3), wherein the optical pickup apparatus records and/or reproducesinformation by converging the first light flux onto the informationrecording surface of the first optical disk, by converging the secondlight flux onto the information recording surface of the second opticaldisk, and by converging the third light flux onto the informationrecording surface of the third optical disk, wherein the objectiveoptical element comprises an optical surface comprising at least twoareas which are a central area and a peripheral area surrounding thecentral area, the central area comprises a first optical path differenceproviding structure, the peripheral area comprises a second optical pathdifference providing structure, wherein the objective optical elementconverges the first light flux which passes through the central area ofthe objective optical element onto the information recording surface ofthe first optical disk so that the optical pickup apparatus can recordand/or reproduce information on the information recording surface of thefirst optical disk, the objective optical element converges the secondlight flux which passes through the central area of the objectiveoptical element onto the information recording surface of the secondoptical disk so that the optical pickup apparatus can record and/orreproduce information on the information recording surface of the secondoptical disk, the objective optical element converges the third lightflux which passes through the central area of the objective opticalelement onto the information recording surface of the third optical diskso that the optical pickup apparatus can record and/or reproduceinformation on the information recording surface of the third opticaldisk, the objective optical element converges the first light flux whichpasses through the peripheral area of the objective optical element ontothe information recording surface of the first optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the first optical disk, and theobjective optical element converges the second light flux which passesthrough the peripheral area of the objective optical element onto theinformation recording surface of the second optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the second optical disk, and whereinthe first optical path difference providing structure comprises a firstbasic structure and a second basic structure which overlap with eachother, the first basic structure is an optical path difference providingstructure which emits a second-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the first basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the first basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstbasic structure, and the second basic structure is an optical pathdifference providing structure which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the first light flux passes throughthe second basic structure, which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the second light flux passes throughthe second basic structure, and which emits a ±1^(st) order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the third light flux passesthrough the second basic structure.
 2. The optical pickup apparatus ofclaim 1, wherein the second optical path difference providing structurecomprises any one structure selected from a group including the firstbasic structure, a fifth basic structure, and a sixth basic structure,the fifth basic structure is an optical path difference providingstructure which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the fifth basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the fifth basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstfifth structure, and the sixth basic structure is an optical pathdifference providing structure which emits a third-order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the first light flux passesthrough the sixth basic structure, which emits a second-order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the second light flux passesthrough the sixth basic structure, and which emits a second orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the third light fluxpasses through the sixth basic structure.
 3. The optical pickupapparatus of claim 1, wherein the objective optical element is a singlelens.
 4. The optical pickup apparatus of claim 1, wherein the objectiveoptical element is a plastic lens.
 5. The optical pickup apparatus ofclaim 2, wherein the first optical path difference providing structurefurther comprises one of a third basic structure, a fourth basicstructure and a seventh basic structure which overlaps with the firstbasic structure and the second basic structure, and the third basicstructure is an optical path difference providing structure which emitsa tenth-order diffracted light flux with a larger light amount than anydiffracted light fluxes with the other diffraction order, when the firstlight flux passes through the third basic structure, which emits asixth-order diffracted light flux with a larger light amount than anydiffracted light fluxes with the other diffraction order, when thesecond light flux passes through the third basic structure, and whichemits a fifth order diffracted light flux with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe third light flux passes through the third basic structure, thefourth basic structure is an optical path difference providing structurewhich emits a fifth-order diffracted light flux with a larger lightamount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the third basicstructure, which emits a third-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the third basicstructure, and which emits third and second order diffracted light fluxwith a larger light amount than any diffracted light fluxes with theother diffraction order, when the third light flux passes through thethird basic structure, and the seventh basic structure is an opticalpath difference providing structure which emits a second-orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the first light fluxpasses through the third basic structure, which emits a first-orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the second lightflux passes through the third basic structure, and which emits a firstorder diffracted light flux with a larger light amount than anydiffracted light fluxes with the other diffraction order, when the thirdlight flux passes through the third basic structure.
 6. The opticalpickup apparatus of claim 5, wherein the second optical path differenceproviding structure further comprises one of the third basic structure,the fourth basic structure and the seventh basic structure which overlapwith any one of the first basic structure, the fifth basic structure,and the sixth basic structure.
 7. The optical pickup apparatus of claim6, wherein the optical surface of the objective optical element furthercomprises a most peripheral area surrounding the peripheral area andcomprising a third optical path difference providing structure, theobjective optical element converges the first light flux which passesthrough the most peripheral area of the objective optical element ontothe information recording surface of the first optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the first optical disk, and the thirdoptical path difference providing structure includes any one structureselected from a group including the third basic structure, the fourthbasic structure and the seventh basic structure.
 8. The optical pickupapparatus of claim 4, wherein the optical pickup apparatus furthercomprises a coupling lens being a plastic lens.
 9. The optical pickupapparatus of claim 1, wherein the first wavelength λ1, the secondwavelength λ2, and the third wavelength λ3 satisfy followingexpressions:1.5×λ1<λ2<1.7×λ1, and1.9×λ1<λ3<2.1×λ1.
 10. An objective optical element for use in an opticalpickup apparatus, the optical pickup apparatus comprising a first lightsource for emitting a first light flux having a first wavelength λ1; asecond light source for emitting a second light flux having a secondwavelength λ2 (λ2>λ1); and a third light source for emitting a thirdlight flux having a third wavelength λ3 (λ3>λ2), and conductingrecording and/or reproducing information using the first light flux fora first optical disk having a protective substrate with a thickness t1,conducting recording and/or reproducing information using the secondlight flux for a second optical disk having a protective substrate witha thickness t2 (t1≦t2), and conducting recording and/or reproducinginformation using the third light flux for a third optical disk having aprotective substrate with a thickness t3 (t2<t3), the objective opticalelement comprising: an optical surface comprising at least two areaswhich are a central area and a peripheral area surrounding the centralarea, wherein the central area comprises a first optical path differenceproviding structure, the peripheral area comprises a second optical pathdifference providing structure, wherein the objective optical elementconverges the first light flux which passes through the central area ofthe objective optical element onto the information recording surface ofthe first optical disk so that the optical pickup apparatus can recordand/or reproduce information on the information recording surface of thefirst optical disk, the objective optical element converges the secondlight flux which passes through the central area of the objectiveoptical element onto the information recording surface of the secondoptical disk so that the optical pickup apparatus can record and/orreproduce information on the information recording surface of the secondoptical disk, the objective optical element converges the third lightflux which passes through the central area of the objective opticalelement onto the information recording surface of the third optical diskso that the optical pickup apparatus can record and/or reproduceinformation on the information recording surface of the third opticaldisk, the objective optical element converges the first light flux whichpasses through the peripheral area of the objective optical element ontothe information recording surface of the first optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the first optical disk, and theobjective optical element converges the second light flux which passesthrough the peripheral area of the objective optical element onto theinformation recording surface of the second optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the second optical disk, and whereinthe first optical path difference providing structure comprises a firstbasic structure and a second basic structure which overlap with eachother, the first basic structure is an optical path difference providingstructure which emits a second-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the first basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the first basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstbasic structure, and the second basic structure is an optical pathdifference providing structure which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the first light flux passes throughthe second basic structure, which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the second light flux passes throughthe second basic structure, and which emits a ±1^(st) order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the third light flux passesthrough the second basic structure.
 11. The objective optical element ofclaim 10, wherein the second optical path difference providing structurecomprises any one structure selected from a group including the firstbasic structure, a fifth basic structure, and a sixth basic structure,the fifth basic structure is an optical path difference providingstructure which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the fifth basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the fifth basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstfifth structure, and the sixth basic structure is an optical pathdifference providing structure which emits a third-order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the first light flux passesthrough the sixth basic structure, which emits a second-order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the second light flux passesthrough the sixth basic structure, and which emits a second orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the third light fluxpasses through the sixth basic structure.
 12. The objective opticalelement of claim 10, wherein the objective optical element is a singlelens.
 13. The objective optical element of claim 10, wherein theobjective optical element is a plastic lens.
 14. The objective opticalelement of claim 11, wherein the first optical path difference providingstructure further comprises one of a third basic structure, a fourthbasic structure and a seventh basic structure which overlaps with thefirst basic structure and the second basic structure, and the thirdbasic structure is an optical path difference providing structure whichemits a tenth-order diffracted light flux with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe first light flux passes through the third basic structure, whichemits a sixth-order diffracted light flux with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe second light flux passes through the third basic structure, andwhich emits a fifth order diffracted light flux with a larger lightamount than any diffracted light fluxes with the other diffractionorder, when the third light flux passes through the third basicstructure, the fourth basic structure is an optical path differenceproviding structure which emits a fifth-order diffracted light flux witha larger light amount than any diffracted light fluxes with the otherdiffraction order, when the first light flux passes through the thirdbasic structure, which emits a third-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the second light flux passes through the thirdbasic structure, and which emits third and second order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the third light flux passes throughthe third basic structure, and the seventh basic structure is an opticalpath difference providing structure which emits a second-orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the first light fluxpasses through the third basic structure, which emits a first-orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the second lightflux passes through the third basic structure, and which emits a firstorder diffracted light flux with a larger light amount than anydiffracted light fluxes with the other diffraction order, when the thirdlight flux passes through the third basic structure.
 15. The objectiveoptical element of claim 14, wherein the second optical path differenceproviding structure further comprises one of the third basic structure,a fourth basic structure and the seventh basic structure which overlapwith any one of the first basic structure, the fifth basic structure,and the sixth basic structure.
 16. The objective optical element ofclaim 15, wherein the optical surface of the objective optical elementfurther comprises a most peripheral area surrounding the peripheral areaand including a third optical path difference providing structure, theobjective optical element converges the first light flux which passesthrough the most peripheral area of the objective optical element ontothe information recording surface of the first optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the first optical disk, and the thirdoptical path difference providing structure includes any one structureselected from a group including the third basic structure, the fourthbasic structure and the seventh basic structure.
 17. The objectiveoptical element of claim 13, wherein the optical pickup apparatusfurther comprises a coupling lens being a plastic lens.
 18. Theobjective optical element of claim 10, wherein the first wavelength λ1,the second wavelength λ2, and the third wavelength λ3 satisfy followingexpressions:1.5×λ1<λ2<1.7×λ1, and1.9×λ1<λ3<2.1×λ1.
 19. An optical information recording and/orreproducing apparatus, comprising: an optical pickup apparatuscomprising: a first light source for emitting a first light flux havinga first wavelength λ1; a second light source for emitting a second lightflux having a second wavelength λ2 (λ2>λ1); a third light source foremitting a third light flux having a third wavelength λ3 (λ3>λ2); and anobjective optical element for converging the first light flux onto aninformation recording surface of a first optical disk having aprotective substrate with a thickness t1, for converging the secondlight flux onto an information recording surface of a second opticaldisk having a protective substrate with a thickness t2 (t1≦t2), and forconverging the third light flux onto an information recording surface ofa third optical disk having a protective substrate with a thickness t3(t2<t3), wherein the optical pickup apparatus records and/or reproducesinformation by converging the first light flux onto the informationrecording surface of the first optical disk, by converging the secondlight flux onto the information recording surface of the second opticaldisk, and by converging the third light flux onto the informationrecording surface of the third optical disk, wherein the objectiveoptical element comprises an optical surface comprising at least twoareas which are a central area and a peripheral area surrounding thecentral area, the central area comprises a first optical path differenceproviding structure, the peripheral area comprises a second optical pathdifference providing structure, wherein the objective optical elementconverges the first light flux which passes through the central area ofthe objective optical element onto the information recording surface ofthe first optical disk so that the optical pickup apparatus can recordand/or reproduce information on the information recording surface of thefirst optical disk, the objective optical element converges the secondlight flux which passes through the central area of the objectiveoptical element onto the information recording surface of the secondoptical disk so that the optical pickup apparatus can record and/orreproduce information on the information recording surface of the secondoptical disk, the objective optical element converges the third lightflux which passes through the central area of the objective opticalelement onto the information recording surface of the third optical diskso that the optical pickup apparatus can record and/or reproduceinformation on the information recording surface of the third opticaldisk, the objective optical element converges the first light flux whichpasses through the peripheral area of the objective optical element ontothe information recording surface of the first optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the first optical disk, and theobjective optical element converges the second light flux which passesthrough the peripheral area of the objective optical element onto theinformation recording surface of the second optical disk so that theoptical pickup apparatus can record and/or reproduce information on theinformation recording surface of the second optical disk, and whereinthe first optical path difference providing structure comprises a firstbasic structure and a second basic structure which overlap with eachother, the first basic structure is an optical path difference providingstructure which emits a second-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the first basicstructure, which emits a first-order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the first basicstructure, and which emits a first-order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstbasic structure, and the second basic structure is an optical pathdifference providing structure which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the first light flux passes throughthe second basic structure, which emits a 0-th-order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the second light flux passes throughthe second basic structure, and which emits a ±1^(st) order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the third light flux passesthrough the second basic structure.